Aeroengine Safety

Figure "Damaging by foreign materials on parts": The damage potential of various surface foulings (Ills. 16.2.2.3-7, 16.2.2.3-8, 16.2.2.3-9, 16.2.2.3-10 and 16.2.2.3-11) can only be recognized upon closer inspection. The following text explains this using a selection of typical examples.

Smearing occurs when there is a firmly bonding connection with the base material. This can occur due to cold welding (galling) under sufficient relative movements and/or pressure. These conditions are given in the following cases:

• Machining with tools such as wire brushes or grinders
• Blasting with contaminated shot
• Clamping the parts with tools and equipment made from potentially damaging materials (Fig. "Damages by metallic surface fouling from finishingfinishing").
• Surrounding the parts with low-melting metals.

The damaging effect primarily occurs through reactions with the base material at high temperatures during subsequent finishing processes and operation. Dangerous diffusion processes and melting lead to local strength losses and/or embrittlement. In extreme cases with sufficiently high tensile stress and wetting conditions, it can result in liquid metal embrittlement (LME, Fig. "Embrittlement by contact with solid metal and melts"), a type of brittle cracking (Fig. "Damages by metallic surface fouling from finishingfinishing"). Additional problems include damage to coatings (e.g. diffusion coatings) that require costly reworking or shorten the oxidation life during operation.

Overlays and residue: Metallic residues such as lead tape are especially dangerous. These are markings similar to adhesive tape. They are used for marking during X-ray testing (Fig. "LME fracture of titanium screw"). Al tape and lead tape are used as overlays during blasting and electroplating. Al tape is also problematic.
Form-fitting residue of low-melting cast alloys can remain in hollow spaces such as the cooling air ducts of blades. Insufficiently removed metal coatings (e.g. silver, Volume 3, Ill. 12.4-14), such as from reworking, can also be dangerous. Their potential damaging effect is similar to that of metallic smearing.
Non-metallic residue forms from dried fluids. These include etching and cleaning baths, as well as foreign contaminants from storage and transport (Ills. 16.2.2.3-2 and 16.2.2.3-6). These contaminants can especially worsen the oxidation behavior of hot parts (Fig. "Residues from finishing in hot parts"). If residue contains halogens such as chlorine, it can cause cracking in titanium parts under tensile stress at temperatures above 450°C (stress corrosion cracking, Ref. 16.2.2.3-1; Ills. 16.2.1.7-8 and 16.2.2.3-16).

Particles and dust: Non-metallic residues from transport and storage, as well as from foreign sources (Ill. 16.2.2.3-2) usually occur as dust or powders. They can also react with the base material. SiC reacts with Ni alloys in unallowable ways at high temperatures (also see Volume 2, Ill. 7.1.4-14).
Loose dust is especially capable of worsening the adhesive strength of thermal spray coatings (microsphere effect, Fig. "Properties of thermal spray coatings by production").
Metallic powders and dusts are created during finishing processes such as grinding, thermal spraying, or laser machining. If these bond with the surface as sparks upon impact, the damage will be especially intensive (Fig. "Damages by splashes of liquid metal").

Spray powders, coating powders, and soldering powders can remain on parts in case of carelessness. The special danger of these highly reactive particles is diffusion and melting of the base material at high temperatures. These usually occur during heat treatment or welding.
Blasting media is the primary type of particle that tends to remain in hollow spaces.

Fluids, greases, and pastes: These are usually auxiliary materials (Fig. "Non specified materials in the finishing process") such as lubricating greases and cooling lubricants (Chapter 16.2.1.1.1). Damage occurs during subsequent coating processes through poor adhesion or delamination of the coatings.

Residue from etching media and soldering pastes: These can cause corrosion damage in combination with condensation water (Ills. 16.2.2.3-15 and 16.2.2.3-17)

Figure "Problems by transferral of undesired parts": An important mechanism through which fouling arrives on part surfaces during finishing is the transferral of media. The actual damaging only occurs during later finishing steps such as heat treatment.

Cleaning and degreasing baths: If the removed fouling, such as greases and oils, float on the surface of the bath similar to grease drops on soup (Fig. "Warning signs of unusable baths"), parts dipped into the bath will be fouled again upon removal (top left diagram, Fig. "Difficult crack detection by fouling on baths"). In this way, fouling can spread like an epidemic. A typical example is silicon as a cause of problems with adhesion and wetting.

Exhaust gases and ventilation systems: If parts and/or transport equipment are stored or transported without sufficient covering, particles and aggressive media can settle on them (top right diagram). Another possibility for damage is the exhaust gas from electroplating if there are no sufficient filters and separators, or if their function is limited for some reason. If acid drops are carried along or acid vapors are trapped by settling air moisture (condensation water), it can cause dangerous corrosion (e.g. intergranular corrosion, IGC). Transport equipment represents a special risk of fouling transferral. Absorbent materials, such as uncovered porous plastics, wooden boxes, or wooden pallets, are especially dangerous. If the material sucks up fouling, it can transfer them to other parts like a stamp. These other parts are then capable of passing on the fouling even further.
Using pressurized air blasts to remove dust and fluids can also be problematic if the functioning of the water/oil separators is limited for some reason.

Condensation of vapors and/or reaction with vapors, as well as dripping of melts at high temperatures: Heat treatment containers, especially those with a vacuum, can release metal vapors. Metal vapors that condense on cooler surfaces of the same part or other parts can cause damages (middle left diagram). Cadmium, lead, bismuth, silver, and aluminum (Fig. "Damages by metallic surface fouling from finishingfinishing"), as well as coating powders (Fig. "Cracking due to metal melt drops") and solders (Fig. "Safety problems by contaminations in furnaces") have an especially high potential for damage. Residue such as X-ray markings (Fig. "LME fracture of titanium screw") and remnants of low-melting cast alloys (Fig. "Heattreatment batch as finishing problem indication") on insufficiently cleaned parts can dangerously reduce the strength of the parts through diffusion, melting, or cracking (LME, Fig. "Operation behaviour of parts by surface fouling").

Contaminants in blasting media: If equipment for peening and abrasive blasting (middle right diagram) is used in a way that contamination of the blasting media is not absolutely prevented, the fouling can be carried to subsequent part batches (Fig. "Shot peening undesirable effects i"). Dangerous wear products from equipment made from unsuitable materials can also be created and transferred to the part in the same work process (Fig. "Fastening of parts during finishing"). A typical example is wear products from lead covering bands that are used to prevent shot particles from entering into hollow spaces.

Abrasive blasting is also used to remove coatings. This means that there is a possibility that these particles may remain on sensitive zones through a charging effect, and cause problems in later finishing steps (Fig. "Oxidation protecting Al diffusion coating problems").

Dust deposits due to carelessness: This risk is increased through improper storage/coverings (Ill. 18-5), poor instructions, low motivation (Fig. "Identifying surface fouling") and/or insufficient work preparation. Opening the lid of a storage container, removing powder, and pouring powder can release powder. This results in dust deposits on parts in the surrounding area (bottom left diagram). These powders include coating powders, spray powders, sintering powders, and solder powders. Due to their low melting point, tendency to diffuse, and wetting characteristics, they are especially problematic for steels, Ti alloys, and Ni alloys. During heat treatment, they can damage both the surface and interior of parts.
If improper methods are used to remove the powder, it can make the problem worse. For example, if a vacuum cleaner with an overly coarse or malfunctioning filter is used, the outlet air can distribute the powder throughout the entire room (bottom right diagram).

Smearing from grinding tools and equipment: Grinding and smoothing tools, especially sanding-type tools, can store wear products. This can result in various materials (e.g. non-ferrous metals, light metals, and Ni alloys) being transferred to different part zones (e.g. coatings) or parts (bottom center diagram).

Equipment or tools (e.g. punches) made from unsuitable material (e.g. copper or brass) can smear onto the parts, causing problems in subsequent finishing processes (e.g. electroplating, etching) or unallowably influencing the material during heat treatment (Fig. "Damages by metallic surface fouling from finishingfinishing").
If tools such as steel brushes or steel wool (!) transfer ferrous wear products onto Ti alloys, it can lead to the embrittlement of weld seams (Fig. "Fouling of Titanium welds").

Figure "Damages by contaminations from production": Fouling and damaging media from finishing can also directly influence the engine. This does not involve contamination of the parts during finishing.
Often, testing rigs are located near finishing shops. Experience has shown that contaminated exhaust air can be carried several kilometers by wind and enter the intake area of an engine. These contaminants collect and build up in the engine, where they can lead to serious damages in the long term. Usually, these damages are caused by corrosion, and especially sulfidation in the hot parts. Air contamination from electroplating should be viewed as being especially aggressive. This type of fouling includes chlorine compounds from etching baths.
In one documented case, the Ni-alloy disks of the low-pressure turbine of test engines showed pitting corrosion after long testing rig runs. It was determined that, during standing times, condensation water formed aggressive electrolytes in combination with chlorine-contaminated air that was sucked up by the engine. The fouling originated in an electroplating shop located several hundred meters away. The electrolytes dissolved silver from the threaded connections on the rotors. This silver solution was centrifuged outward when the engine was started up. It vaporized at the high operating temperatures, and the precipitated silver caused sulfidation with pitting corrosion-like damages in critical disk zones (Volume 3, Ill. 12.4-14).
In another case, lacquer vapors were sucked up by an engine on a testing rig during an acceptance test. The deposits necessitated disassembly and complete cleaning of the compressor, which resulted in a very high cost (Volume 1, Ill. 5.5-1).

Figure "Non specified materials in the finishing process": Many problems with surface fouling in finishing can be traced back to auxiliary materials. The damaging potential of transferred auxiliary material residue will be shown using typical examples.

Lubricants: Machine tools require lubrication with grease or oil on sliding surfaces, bearings, shafts, etc. If parts are fouled with these lubricants and not sufficiently cleaned, there is a danger
that the parts could be damaged in subsequent finishing steps, such as during coating (Ref. 16.2.2.3-5) and heat treatment. A similar problem with residue from paint marking is shown in Fig. "Unsuitable paints and markings". In nickel alloys under tensile stress, MoS₂ additives can cause dangerous cracking and/or oxidation (sulfidation). Other oil additives can also lower the oxidation resistance and thus affect later operation.

Anti-foaming agents/silicon oils (Fig. "Risks from coolong lubricants"): As fouling, these compounds influence the wettability. This can have serious consequences:
The adhesive strength of coatings such as lacquers and the coating buildup can be compromised. If the strength between the layers of fiber-reinforced polymers (FRP) decreases, it will result in delamination-sensitive flaws that are very difficult to detect using non-destructive testing methods.

Crack detection with the aid of penetrant (Fig. "Difficult crack detection by fouling on baths") may become less sensitive. This effect is suspected to exist, but testing results are inconclusive. However, since the suspicion exists, thorough cleaning of potentially affected parts is a necessity. This means that the risk is limited to parts that were unconsciously affected and installed in engines.

Cooling lubricants (CL, Chapter 16.2.1.1.1): Especially difficult to cut materials, such as Ni and Ti alloys, may require cooling and cutting fluids for better cutting performance. There may be a risk that these fluids could have an aggressive and damaging effect in later finishing processes. High-strength titanium alloys are especially sensitive to cracking (SCC) under the influence of chlorine. This is probably also true of other halogens such as fluorine. Because of this, if possible, cutting fluids containing chlorine and sulfur should generally be avoided in finishing processes due to the risk of them being transferred (Ref. 16.2.2.3-2). In the case of nickel alloys, sulfuric components at high temperatures (welding, heat treatment) can promote sulfidation during operation.
If oils containing sulfur act on silver-plated parts such as bolts, nuts, and sliding surfaces, dark tarnishing to the point of blackening reveals the formation of silver sulfide. This can increase the coefficient of friction considerably, and worsen the sliding properties. This means that the required preloading force of threaded connections may not be reached using the specified tightening torque. Poor sliding properties of silver coatings can promote operating damages through hot running and galling. A similar relationship was observed in the 1960s in the silver-plated slide shoes of axial piston fuel pumps. Silver sulfide created by microbacteria in turn created aggressive sulfur compounds in the fuel tanks, which caused catastrophic pump damages after short operating times.
Lubricants are also used in reforming. They are applied to the parts and/or tools to minimize wear and friction forces (Ref. 16.2.2.3-4). The lubricants make it possible to reform materials that are very difficult to deform, such as high-strength titanium alloys. During the reforming process at high temperatures (e.g. intermediate annealing), these lubricants combine with the part surface and are then very difficult to remove. Lubricant residue such as fatty acid amides or softening agents can cause flaws in subsequently applied lacquers (Ref. 16.2.2.3-5). The reforming process is often done at high temperatures, and/or heat treatment is done between the single reforming steps. This creates a risk that components of the lubricant (e.g. MoS₂ ) may have a damage-promoting reaction with the surface. The selection and approval of these lubricants must take this danger into account. In some cases, suitable aftertreatment of the parts may be necessary. If the parts are to be coated, such as with lacquer, enamel, electroplating, or thermal spraying, the surface must be suitably treated (Ref 16.2.2.3-4). Experience has shown that even tiny amounts of lubricant residue (top atom layer) can lead to craters, cracks, separation, and wetting problems in lacquers.
Secondary ion mass spectroscopy (SIMS, Ref. 16.2.2.3-5) has proven effective for detecting such minor amounts of fouling.

Cleaning agents: Even remnants of these materials can compromise coatings, especially lacquers. This was seen in the case of traces of a synthetic detergent (sodium lauryl sulfate) remaining during the production process (Ref. 16.2.2.3-5).

Releasing agents usually contain PTFE (Teflon®) or silicon compounds. They are especially used in the production of fiber-reinforced synthetic resin parts. They are applied to the molds and enable easier release of the part. These media can be transferred by careless spraying, cleaning rags, or cleaning baths. This then results in poor wettability and adhesion problems during coating processes. The bonding of layers of fiber-reinforced parts is weakened, promoting delamination.

If Teflon residue is sufficiently heated, such as during heat treatment, it will create fluorine. Similar to chlorine, it can be assumed that the fluorine will pose a risk for stress corrosion cracking in titanium parts under sufficient tensile stress.

Covering materials: In finishing, masking/covering materials are necessary for processes such as soldering and diffusion coating. They prevent undesired coating of certain surfaces. If residue of these covering materials remains after the finishing step, or if covering materials are transferred to surfaces that are to be coated or other parts, it will result in flaws.

High-temperature lubricants are used on heat treatment equipment and threaded connections in hot parts in order to make damage-free removal possible later. These media contain dry lubricants such as Cr oxides and hexagonal boron nitride. It is feared that this type of fouling can change the friction conditions in friction welding (Fig. "Process of fly wheel inertia or friction welding") in a way that results in bonding flaws. This danger could also be present in the case of lubricants containing graphite.
Hand washing pastes, skin creams, and other sanitary media: If these contain materials such as silicon, they present similar threats to the integrity of the parts as do silicon-based anti-foaming agents and releasing agents. The danger of transferral is primarily due to prints from unprotected hands.

Figure "Unsuitable paints and markings": A special field of damage-relevant fouling is the marking of parts with markers, chalks, and paints. Fundamentally, the marking materials must be approved for the specific application. However, it is possible that this approval only applies to a specific process step, and the use of the marking materials in other, subsequent steps is questionable. In this case, the parts must of course be sufficiently cleaned.
The degree to which finishing processes and part characteristics are affected by markings cannot always be accurately estimated. If new methods or new auxiliary materials such as cleaning agents are used (e.g. for environmental reasons), it is possible that markings will not be sufficiently removed. In this case, unexpected consequences cannot be ruled out.
Porous materials such as coatings or metal felts can absorb the paint from stamps so that it has a damaging effect both inside and on the adhesive layer.

Figure "Safety problems by contaminations in furnaces": A special problem is the influence of foreign metals (Ills. 16.2.2.3-10.1 to -11).
This can occur when metals with lower melting points than the base material are in a wetting contact with the metallic part surface at high temperatures. Therefore, dangerous damaging will not occur in all cases of metallic fouling on heated parts. Oxidation can prevent the necessary metallic contact. This is true for oxides on the part surface, as well as for situations in which the fouling oxidizes before melting when it is heated.
Therefore, specimen tests that do not show any damaging effects should be viewed skeptically. In this case, the test may not simulate the necessary unfavorable combination of damage-relevant influences.
The following damage mechanisms are primarily active between metallic surface fouling and the base material:

Melting in connection with the formation of low-melting phases. This creates the danger of clear, deep-acting strength losses and embrittlement.

Micro and macro cracking (liquid metal embrittlement, LME) usually require sufficiently high tensile stress while the melt acts. The melt then shoots into the grain boundaries (Fig. "Operation behaviour of parts by surface fouling"). The tensile stresses can form as thermal stress during heating or cooling. However, they may also have been present in the part as residual stresses (e.g. from forging) before heating.

Diffusion: Even before melting of the fouling, it is possible for the base material to be damaged through diffusion. Depending on the material, this can already happen at relatively low temperatures. For example, cadmium damages titanium alloys at around 260°C (Fig. "Damages by metallic surface fouling from finishingfinishing"). Diffusion can decisively reduce strength and/or cause embrittlement. If diffusion prevents a protective oxide layer from forming, then dangerous corrosion can occur in subsequent finishing processes such as etching.
It is also possible that damaging metals could be transferred as vapors, such as when a metal vapor forms from fouling in a vacuum oven, and then settles on the part (Fig. "Operation behaviour of parts by surface fouling").

A decisive factor for damage can be temperature changes in finishing processes or operation that create high thermal stress with plastic deformation of the surface (thermal fatigue). If a protective oxide layer is torn open beneath a metal melt, it can result in melting and/or cracking in the base material. This can make it possible for metal melts to even attack oxidized parts of the type expected in operation (Fig. "Heattreatment batch as finishing problem indication"). The required metallic contact between the fouling and the base material can also occur through smearing (Ref. 16.2.2.3-12), for example, during handling. If no separating oxide layer forms during subsequent heating, then a damaging situation is given.

If heating occurs in a vacuum (heat treatment, high-temperature soldering) or in an inert (heat treatment) or caustic atmosphere (diffusion coating), protective oxide coatings cannot be expected.
Some typical metals that have considerable damage potential as fouling are (Fig. "Damages by metallic surface fouling from finishingfinishing"):

• Low-melting alloys such as embedding masses made from Pb, Bi, Sn alloys, or residue from lead tape used for masking or marking (X-ray testing; Fig. "LME fracture of titanium screw").

• Silver and silver alloys from hard solders or coatings (Fig. "Embrittlement by contact with solid metal and melts").

• Soldering powder for high-temperature soldering, which has its melting point lowered through additives such as boron (Ills. 16.2.1.4-14 and 16.2.2.3-5).

• Coating powders for diffusion coating with high Al contents (Ills. 16.2.1.8.1-3 and 16.2.2.3-13).

Dangerous fouling may already be present before the part enters the oven. However, it may also arrive on the part in the oven. This risk increases if the oven is used for various finishing processes such as vacuum annealing and soldering. The bottom diagram shows a case in which a pinhead-sized drop of silver solder caused through-going cracking in a forged disk one centimeter thick. The solder drop evidently originated in an earlier soldering batch and was caught in the porous graphite insulating mats of the oven or on the charging rack (top diagram). This case also demonstrates the importance of housekeeping (Ills. 16.2.2.3-6 and 16.2.2.3-20).

Figure "Surface fouling by external sources": Experience has shown that construction work must often be done in finishing halls in the context of reconfigurations, repairs, changed processes (moving machines) and the assembly of new production facilities and machines. Often, finishing must continue in the same room during the construction work. In these cases, there is an increased risk of damage-relevant fouling of parts. The remedy is especially careful housekeeping. This includes

• appropriate transporting containers for the parts (Ill. 18-5)
• coverings (Ill. 18-5)
• temporal and physical separation (e.g. provisional) of the construction work and the finishing area
• Increased awareness for signs of fouling such as dust and spots

Metal drops from welding, grinding, and cutting must be seen as being especially dangerous (Ills. 16.2.2.3-3 and 16.2.2.6-5). In this case, there is a danger of reactions and the induction of high tensile stresses resulting from rapid local heating of the metal surface. This can dangerously reduce the dynamic fatigue strength and the cyclical life span.
Ferrous grinding dust can cause embrittlement and cracking in welds on titanium alloys.
Illustration 16.2.2.3-1 describes the dangerous effects of lacquers, oils, and dust.

Figure "Finishing process particles in flow systems": Particles in cross-sections with a throughflow in the fuel (top left diagram), oil (bottom left diagram), and air (right diagram) systems can lead to part failure with extreme consequential damages. The damage cause is a reduction, blockage, and/or diversion of the flow.
If cooling air is required, the overheating damage on hot parts (Fig. "Damages in cooling air systems by finishing coused fouling") is to be expected. Roller bearings and slide seal rings are doubly affected by reduced oil flows. First of all, there is insufficient cooling, and secondly, poor lubrication results in greater heat development. Even the deflection of a jet coming out of a nozzle can affect its function (lubrication) and/or overheat part areas such as combustion chambers and housings due to the angled burning of the flame.
Particles on the bearing races lead to failure due to race fatigue in roller bearings. If abrasive particles enter between the seal surfaces of floating ring seals, it will compromise the seal and most likely result in an oil fire.

Figure "Residues from finishing in hot parts" and Figure "Damages in cooling air systems by finishing coused fouling": The diagram shows damage possibilities on the inside and outside of a cooled turbine blade. Fig. "Damages in cooling air systems by finishing coused fouling" provides more detail regarding inner damages (cooling air zone). The operating behavior, especially the life-determining characteristics, can be affected by fouling in various ways.

Fouling from the finishing process can, for example, compromise cooling by blocking the cooling air bores with particles. In the depicted case, potential threats to the operating behavior from fouling are shown using the example of a cooled turbine rotor blade. Based on their damage mechanism, they can be grouped into the following categories, which can influence one another:

Damages that already occurred in the finishing process due to fouling. One type of damage is the loss of strength. Examples include unallowable corrosive attack around the fouling or reactions with the fouling during heat treatment (Ills. 16.2.2.3-1 and 16.2.2.3-5).

Increased operating temperatures: If one remembers that an increase in operating temperatures, which are always near the physical limits of the materials, of about 15°C will result in a 50% reduction in creep life, it is easy to recognize the effects of even apparently minor disruptions of the cooling. To make this even clearer, it should be pointed out that +30 °C results in a 75% reduction of creep life, while at + 45 °C creep life is reduced to one tenth.

The following influences can be expected to worsen the cooling and result in local temperature increases in the part (see right diagram in Fig. "Damages in cooling air systems by finishing coused fouling"):

• constriction or blocking of the cooling air ducts through finishing shavings, splashed metal (e.g. from laser boring), spray powders, blasting powders, diffusion coating powders, and core residue. In this context, the dust removal bores (Fig. "Damages in cooling air systems by finishing coused fouling") are especially important. They are located at the end of the radially arranged cooling air bores. These are usually very small bores. Due to their function, they can easily be blocked by larger particles such as shavings or agglomerates. Even if the cooling air flow through the whole bore, which has many holes for cooling films and impingement cooling, appears to be small, changes can cause the creep life to drop considerably below the guaranteed values.
  

• Increased oxidation due to etching agent residue and other media that prevent protective oxide layers from forming (Fig. "Damages in cooling air systems by finishing coused fouling"). Oxides in the cooling system have two negative effects. They constrict cooling air ducts and their insulating effect prevents the necessary heat removal. This increases the temperature of walls, causing the oxide layer to grow, and the damaging effect accelerates.
  

• Strength losses and/or embrittlement: Both effects compromise both the (static) creep life and the life span in case of thermal fatigue. Plastic deformations under these operating loads can cause cracks to form early in embrittled areas. Typical strength-reducing influences include:
  

• Reactions of foreign metals (LME, diffusion, Fig. "Damages by metallic surface fouling from finishingfinishing") and other particles such as silicon carbide (SiC) with Ni alloys.

• Intergranular (crack-like) corrosion around etching media residue (Fig. "Damage risks by etching baths").

• Weakening of load-bearing walls as a result of increased oxidation (sulfidation). This is especially promoted by deposits containing silver and/or sulfur (Fig. "Operation behaviour of parts by surface fouling"; Volume 1, Ill. 5.4.5.1-4 and Volume 3, Ill. 12.4-14).

• Increases of the LCF loads: Adhering particles and material damages on blade roots under high LCF stress. This can worsen the contact conditions precisely in the damaged zones, creating dangerous local stress peaks.
  

Changes in the part function and other life-determining effects: If, for example, the oxidation resistance of the diffusion protection coating on the face of a turbine blade is worsened by fouling, material removal through oxidation can change the profile significantly and at least worsen the efficiency of the blade.

Figure "Embrittlement by contact with solid metal and melts" and Figure "LME fracture of titanium screw" (Examples 16.2.2.3-4 and 16.2.2.3-5): Metallic fouling can decisively worsen the strength properties of parts and is therefore a safety hazard. The damage mechanisms are described in the following.

There are several different terms for the same damage mechanism: liquid metal embrittlement (LME), liquid metal induced embrittlement (LMIE), strain-induced stress corrosion cracking in liquid metals. There are evidently multiple requirements for these damages to occur:

• sufficiently high tensile stress
• wetting conditions, i.e. the melting temperature of the fouling must be exceeded, and there must be no separating layer (e.g. oxide coating; Ref. 16.2.2.3-9)
• sensitive material or material state
• cracking (corrosion) relevant media; i.e. metal melt

Conditions for liquid metal embrittlement can occur in many different ways (bottom diagrams).
The requirement for metallic contact between the melt and the base material is met by the surfaces of nickel and titanium alloys, which are metallic and not noticeably oxidized, at least during finishing. This is also true of heat treatments and processes (e.g. coating) in vacuums or under
reducing conditions. Smearing promotes continuous metallic contact, in spite of subsequent oxidation. This type of smearing is cold welding (galling). Typical causes are tools made from unsuitable materials, joining processes with coated fitting surfaces (e.g. silver), and blasting media as fouling.
Even beneath thick coatings such as silver or Cd coatings on Ti or Ni alloys, a metallic contact can remain over longer operating times (Examples 16.2.2.3-3 and 16.2.2.3-4).
If the separating oxide layer is damaged beneath the melt by a scratch, it will result in LME. Scratches can be created by tools or relative movements between contact surfaces. Examples include thermal strain-dependent relative movements between the contact surface and the charging rack or equipment.
Another possibility is the breaking open of oxide layers and the base material surface during cyclical plastic deformation (LCF loads, thermal fatigue). This mechanism can also explain LME damages that occur after longer operating times (Example 16.2.2.3-2, top left diagram).
If a drop of melt falls onto a metallic surface with sufficient kinetic energy, thin oxide layers (e.g. during annealing in cover gas, in a vacuum, or in a reducing atmosphere) can be penetrated by the drop impact (Fig. "Damages by splashes of liquid metal"), which will result in creation of the metallic contact necessary for damage to occur (top right diagram, Fig. "Safety problems by contaminations in furnaces").
Even small amounts of metal are sufficient to cause liquid metal embrittlement (Ref. 16.2.2.3-12). The fouling does not necessarily have to be pure metal. Even metal pigments in coating materials are sufficient.

Embrittlement through metallic contact in the solid state (solid metal induced embrittlement, SMIE): For embrittlement to occur during contact with metallic surface foulings, it is not necessary for the fouling to be in a melted state. There are also cases in which solid-state metal contact can cause embrittlement and cracking. This damage mechanism applies to the bursting of silver-plated nuts at operating temperatures below the melting point (top center diagram, Ref. 16.2.2.3-8).

Melting (Fig. "LME fracture of titanium screw"): Damage can occur if a metal melt is in contact with the base material, even if the required tensile stresses are not present. This damage occurs through the formation of low-melting phases, which can also cause the base material to melt. The strength values of these melts are usually considerably worse than those of the base material. Often, the melted area is so embrittled that there is a risk of immediate cracking under operating loads in the LCF range. If, during cooling, sufficiently high shrinkage stresses are created between the solidified melt and the surrounding base material, it can cause cracking (e.g. hot cracks). Depending on the amount of fouling, even thick cross-sections (centimeters) can be irreparably damaged.

Diffusion processes: The base material can be damaged even if the tensile stresses are not high enough and the melting temperature of the fouling is not exceeded. This occurs when diffusion processes take place.

Figure "Titanium rotor fusing by lead during heat treatment" (Refs. 16.2.2.3-16 and 16.2.2.3-10): In several similar cases, combustion chamber outer casings (CCOC) fractured during takeoff. At this point, the combustion chamber pressure and loads on the pressure casing are especially high. Investigations revealed that, contrary to regulations, there was an Ni-Cd coating in the holes for the threaded connection of the rear flange. This coating is applied to steel housings for corrosion protection, and consists of a nickel coating as an intermediate layer for the Cd coating.
However, if this protective Ni coating fails or if Cd is directly precipitated onto the steel, it will result in LME at the high temperatures of the combustion chamber casing.
This situation was given following overhauling of the flange bores. Cracking (bottom detail) and crack growth caused the housing to burst. Hot gases and the explosion pressure led to fuel escaping and igniting. However, safe evacuation of the aircraft was successful in the reported cases.

Figure "Damages by metallic surface fouling from finishingfinishing" (Ref. 16.2.2.3-8): This table shows metal combinations found in engine production that, under unfavorable conditions, can result in liquid metal embrittlement (LME, Fig. "Residues from finishing in hot parts") and damage through contact with solid bodies (SMIE). These materials are primarily titanium alloys, nickel-based alloys, and high-alloy steels. Low-melting metallic surface fouling consisting of or containing cadmium (Cd), silver (Ag), lead (Pb), and bismuth (Bi) are especially dangerous.

Cadmium was often used in older engine types as corrosion protection for rusting steels (Example 16.2.2.3-3, Ill. 16.2.2.3-10.4, and Ref. 16.2.2.3-17). Cadmium was transferred to corrosion-resistant materials via baths or by smearing. In rare cases, it was part of an intentional coating (Example 16.2.2.3-4). Today, use of Cd is avoided if possible, due to its toxicity and role in hydrogen embrittlement.

Silver and silver alloys are used on threaded connections to improve the rubbing conditions (low friction torque in threads) during tightening and loosening. In operation, silver can be transferred during standstill to other parts through corrosion (Volume 3, Ill. 12.4-14). Sliding surfaces and fitting surfaces can transfer silver to other parts.

Lead and bismuth: Lead is used in a relatively pure form in coatings and lead tape for masking (electroplating, blasting) and marking (X-rays, Fig. "LME fracture of titanium screw"). It can be transferred from coatings onto fitting and sliding surfaces (e.g. shafts). Blasting media fouled by lead masking bands, or remnants of these bands, can also contaminate surfaces (Fig. "LME fracture of titanium screw").
Lead and bismuth are used in low-melting alloys that are poured to fasten parts. In this case, there is a risk of residue remaining after the process, especially in hollow spaces of cooled turbine blades and as smearing (Fig. "Damages in cooling air systems by finishing coused fouling" and Example 16.2.2.3-2).

Copper, brass, and bronzes: These are known as causes of liquid metal embrittlement in steels. The melting of copper (e.g. electrode or coating) occurs during welding (Ref. 16.2.2.3-17).
Copper is also used as a solder for assembled compressor stator vanes and housings (Fig. "Brazing applications in engines") in older engines. Punches made from copper or bronze can create dangerous smearing.

Gold is used in engines, albeit rarely, as a reflective coating to keep operating temperatures within acceptable limits. Due to its high toughness and low soldering temperatures, gold solder is used in some assembled guide rings made from nickel alloys near the compressor outlet.

A special, if apparently indirect danger, is posed by smearing from gold jewelry. Finger rings made from gold can transfer gold to parts during handling. For this reason, finger rings should not be worn during this type of work.

Figure "Surface fouling affecting finishing and assurance": Surface fouling rarely has damaging effects in the finishing process in which it originates. It is more common for the damage to occur in subsequent finishing steps, and to be detected even later (top diagram). Even the compromising of a non-destructive testing method can be classified as damaging. A typical example is fouling containing chlorine on titanium parts. Only later, at processing temperatures above 450°C, does cracking occur. Therefore, it requires a great deal of expertise to be able to analyze the damage symptoms and finishing sequence and determine the cause and time (finishing step) at which the fouling occurred. The diagram shows typical process-specific problems and damages that are related to fouling from other finishing steps, handling (Ills. 16.2.2.3-16 and 18-4) and storage. It is easy to recognize that the time and location at which fouling occurs are often not identical with the damage symptoms.

The adhesive strength of coatings that are

• galvanically precipitated
• applied as lacquers
• thermally sprayed
• glued
• sintered
• vapor depositedis especially sensitive to fouling on the surface to which they are applied.

Insufficient adhesive strength can immediately reveal itself through flaking or blistering. It may also only reveal itself after additional finishing steps. If the coating has not yet separated from the base material, non-destructive detection of adhesion problems is usually not possible (Ills. 16.2.1.6-10, 16.2.2.8-5 and 16.2.2.8-6). This type of fouling primarily results from carelessness during handling, transport, and storage. It can disrupt the coating process through the formation of oxide layers at sufficiently high temperatures (welding, heat treatments), as well as by worsening the wettability.

Coating structures: One example is the compromising of diffusion coatings. Ceramic and metallic residue or oxides can locally prevent diffusion. This influences the composition and thickness of the coating, and lessens the protective effect.

Strength and toughness of the base material Splashes of metal melts are created by welding (EB, friction welding), while hot particles are created by grinding, separating, and high-speed machining. They have damaging effects on the part surface (Fig. "Damage potential of hot metal particles"). If splashes have been removed in an uncontrolled process, subsequent non-destructive detection of damage (Fig. "Damages by splashes of liquid metal") becomes considerably more difficult.
If fouling acts like metal melts (LME, SMIE) and reacts with the base material, it can be expected to cause significant strength losses and embrittlement (Ills. 16.2.2.3-10.1 and 16.2.2.3-10.2).
Stress corrosion cracking in titanium alloys occurs in titanium alloys under the influence of fouling containing chlorine (Fig. "Chlorine in process baths causing stress corrosion").
Increased hydrogen absorption and embrittlement (hydrogen embrittlement) around fouling usually occurs in galvanic processes and in etching baths. One example is the effect of ferrous deposits during the etching of titanium alloys (Fig. "Hydrogen embrittlement by etching process"). These deposits include grinding particles and rust from iron wear products (wire brushes, steel wool, shot peening). Smearing from tools and equipment is another source. The iron can embrittle welds on titanium alloys.

Compromising the sensitivity of non-destructive testing: A typical example is poor wettability due to fouling such as silicon compounds (e.g. from cutting fluids, hand creams, and releasing agents), which can compromise the effectiveness of penetrant testing (Ills. 17.3.1-2 and 17.3.1-3).
The detectability of segregations during etching tests such as blue etch anodizing (BEA, Fig. "Flaw causing titanium rotor fracture i") of titanium alloys or segregation etching on Ni-based forged alloys (Fig. "Segregation distributions in billets") can understandably be negatively affected by fouling.

Strength of soldered and welded joints: Examples include flaws such as pore formation through vaporizing contaminants in the welding gap (EB welding, Fig. "EB welding flaws") and laminar bonding flaws. Diffusion welds react especially sensitively to fouling that prevents metallic contact between the welding surfaces, thereby preventing diffusion (Fig. "Diffusion weld of a ‘dual property’ part").
In friction welds, it is suspected that bonding flaws are caused by changes in the friction conditions by fouling with the properties of high-temperature lubricants (e.g. hexagonal boron nitride or graphite).

Figure "Operation behaviour of parts by surface fouling": In exceptional cases, damage can also enter into engines (Fig. "Stress corrosion cracking by process baths and hand sweat"). There are various reasons for this:

• Failure of the testing process
• Fouling during transport and handling of the tested part
• The actual damage only occurs under operating influences

Damages that already occur during finishing can negatively affect both the strength and function of parts.

Cracking, such as liquid metal embrittlement, presents a direct danger of catastrophic failure (top right diagram).

Embrittlement of titanium parts during etching presents a danger of dynamic fatigue cracking in operation-specific notches (erosion). The cause is rust (top left diagram).
Formation of silver sulfide on silver-plated bolts and nuts through contact with sulfurous media (e.g. cutting oil). This affects the tribological behavior by increasing the coefficient of friction (middle right diagram).

The following considerations are limited to fouling with a damaging effect that only occurs under operating conditions (bottom frame). This means that the operating temperature is decisive for the damaging effects of fouling.
Cracking as the result of a reaction between metals (LME and SMIE, Ills. 16.2.2.3-10.1,.2 and 16.2.2.3-11) can also occur during operation. This is especially likely in the case of smearing and firmly adhering particles, when the material connection prevents the formation of a protective intermediate oxide layer.

Increased oxidation resulting from etching agent residue (Fig. "Damages in cooling air systems by finishing coused fouling") has a thermal insulation effect and also constricts cooling air ducts. This unfortunate combination causes the part temperature to rapidly rise, shortening the part life. Blockages with particles from the finishing process have similar effects. If these contaminants promote grain boundary corrosion (Fig. "Damage risks by etching baths"), it will further reduce the strength levels.
Residue from etching agents and flux (Fig. "Corrosion of brazed joints") can have similar effects, and can cause corrosion damage in case of alternating condensation water formation and heating (Fig. "Finishing related corrosion fracture of pipe connections").

Figure "Heattreatment batch as finishing problem indication" (Ref. 16.2.2.3-6): This flight accident was traced back to the fracturing of several turbine blades in the first high-pressure stage (bottom right diagram). All fractured blades came from the same heat treatment batch, which had already been connected with previous similar damages. 1.4-1.9 ppm of the element bismuth was discovered in the base material of the blades. These minimal traces of bismuth were responsible for the reduced life span of the blades.
The main question was how the bismuth entered into the turbine blades. There were three likely possibilities (bottom frame):

• Smearing of a low-melting cast workholding material that was used to hold the blades during machining (Fig. "Embrittlement by contact with solid metal and melts").
  

• Residual cast workholding material on and/or in the blade (Fig. "Finishing process particles in flow systems").

• Vapor-deposited cast workholding material during a finishing process (heat treatment in a vacuum, diffusion coating, high-temperature soldering).

Unfortunately, the available literature does not indicate whether the part had traces of bismuth all over, or if these were concentrated in the fracture zone. It is also unclear whether the distribution was even, or whether there were concentrations at structural characteristics such as the grain boundaries.
Material fouling limited to the damaged area would indicate smearing or residue in the cooling air ducts. However, the fact that the affected parts were apparently limited to a heat treatment batch would contradict this theory of the method by which the bismuth fouling occurred. This can hardly be plausibly explained by coincidental residue and smearing.
If the blade is fouled across a larger area, it would indicate the occurrence of diffusion in relation to vapor deposition. This type of process might occur in a vacuum or cover gas oven, in which bismuth fouling vaporizes and settles on the parts. This theory is supported by the fact that only one heat treatment batch was affected.

Figure "Finishing related corrosion fracture of pipe connections" (Ref. 16.2.2.3-13): The literature about this damage incident states:
“The first two losses (of modern military fighters) occurred in 1997 and seem to have been caused by the rupture of a flexible hose. `The hoses had been contaminated by chlorine during the manufacturing process, and this weakened the braids in the hoses once the engine entered service'.”
Details have been depicted by the author on the basis of his experience. It can be assumed that the chloric residue combined with condensation water to form a corrosive medium (also see Fig. "Corrosion by remains of flux") that, especially at operating temperatures, had a very concentrated effect on the stainless steel wires (usually made from CrNi18 9) of the supporting sleeve. Under a microscope, the corrosive attack probably looked like that shown in the detail.

Figure "Stress corrosion cracking by process baths and hand sweat": Two examples of stress corrosion cracking related to media from finishing. In the case in the top diagram, chloric residue from a degreasing bath caused stress corrosion cracking during subsequent heat treatment and high tensile stresses (welding stress).
The bottom diagram shows the origin of the blade fracture that occurred during operation as a result of stress corrosion cracking. Dried hand sweat in an area with high tensile residual stresses from forging caused the cracking. Even this minor amount of chlorine in the form of common salt residue is enough to cause cracking.

Figure "Corrosion by remains of flux": Residue from etching agents and corrosive media from the finishing process, such as etching baths, fluxes, and soldering pastes, can have damaging effects during later storage and/or under operating conditions (Fig. "Heattreatment batch as finishing problem indication").
Brazing with Cu- or silver-based solders was used on oil lines made from 18 8 type steels and, in some older engine types, on assembled compressor stator vanes made from 13% Cr steels (Fig. "Brazing applications in engines").
In the depicted case, a dynamic fatigue fracture of an oil nozzle (right diagram) occurred in an accessory gearbox (left diagram). The broken nozzle became trapped between the gears and caused extensive consequential damage. The dynamic fatigue fracture started at the transition of the soldered joint, which showed intergranular corrosion (detail) that was traced back to aggressive flux residue.
This type of corrosive attack is possible if the dried fouling combines with condensation water or increased air moisture to create a watery electrolyte (Fig. "Operation behaviour of parts by surface fouling"). Corrosion will be promoted by element formation between the solder and base material. In addition, in some materials, soldering can result in an unfavorable structural state (sensitization) that promotes grain boundary corrosion (intergranular corrosion, IGC).

Figure "Measures preventing fouling": Preventive measures can be taken during the finishing process to deal with questionable surface fouling on parts. The effectiveness of these measures often lies in their combination. In the following, preventive measures were categorized according to individual steps of a typical finishing process.

“1”: Effective housekeeping is especially important. Housekeeping refers to cleanliness and order, as well as the necessary discipline to maintain this. It requires understanding and awareness from all workers. This includes giving all affected personnel plausible explanations for these requirements. This attitude can be fostered by suitable training courses, even on-location with memorable examples.
In order to realize the desired housekeeping conditions, a zero tolerance policy similar to that successfully implemented in large US cities (New York) is a promising approach. This is based on the experience that silently tolerating a cigarette butt being thrown on the ground will soon result in a mountain of garbage around it. Naturally, a suitable infrastructure must be in place to realize this. This includes a sufficient number of suitable garbage containers, transporting containers that cannot be used for anything but their intended purpose, proper handling, and proper storage of the parts.

“2”: Even the delivery of the parts to the individual processing steps is of decisive importance in being able to detect fouling in time (Fig. "Detecting fouling in hollows"). There are several reasons for this:
If the personnel are sufficiently experienced, there is a high probability that they will notice unusual changes in the parts. This could be in their own best interest, since problems and rejects will otherwise be attributed to their “own” finishing processes, with unpleasant consequences. If fouling is detected sufficiently early, the risk of high scrapping costs can be minimized. In addition, there is a higher probability that the cause can be rapidly found and specific corrective measures taken.

“3”: In order to estimate the risks of potentially affected or even delivered parts, it is necessary to understand the damage that has already occurred, and also to identify the fouling. There are specific laboratory testing methods for this purpose, especially microanalysis using SEMs (Fig. "Scanning electron microscopy (SEM)"). If possible, especially in the case of expensive parts, the tests are done in a non-destructive manner. The identification of an active damage mechanism, as well as the extent of damage, are prerequisites for the development and approval of any reworking and repairs (Fig. "Systematic analysis of problems i").

“4”: The monitoring of the finishing process, i.e. the parts and auxiliary materials, by experienced and motivated personnel is of decisive importance (Fig. "Quality assurance by process monitoring"). Unusual changes in parts (e.g. color, sheen, stains) and/or unusual behavior of baths (e.g. bubbling, discoloration, effectiveness) and machines (strange noises, wear, shavings) can be important indicators.

“5”: If fouling and/or unusual surface changes have been confirmed, an investigation by specialists is necessary in order to identify the damage for risk assessments and possible reworking. The person the specialists report to, or at least the chain of information, should be clearly determined and understood. This is especially true for finished parts that are to be delivered following reworking.

“6”: In order to prevent fouling, it may be necessary to implement cleaning processes at suitable places in the finishing sequence. These processes must be selected, tested, and approved by the responsible specialists. This is absolutely necessary in order to prevent unexpected new damage-relevant effects (e.g. etching damage or damage to coatings) from occurring.

Figure "Detecting fouling in hollows": A special problem is the detection of finishing-related fouling in poorly visible hollow spaces. This includes cooling air ducts in turbine blades, integrated oil bores in gearbox housings, and branching pipes as are typical for oil systems. For this reason, special attention should be paid to fluids and particles falling or flowing out of parts during handling. Reaction zones and deposits (stains) around bores that lead into the part are also important indicators (top left diagram).
Constrictions and blockages can be detected with the aid of flow metering, perhaps in combination with thermography (middle diagram).
In order to detect internal fouling with X-rays, it is necessary to weaken the X-ray power sufficiently to match the part walls.
If fouling can be dissolved by an otherwise non-damaging bath, it may be possible to detect fouling on the parts on the basis of contaminants in the bath (right diagram). This process is used to detect extremely small amounts of residue of low-melting workholding materials. For the material bismuth (Bi), the detection limit is 0.02 ppm (Ref. 16.2.2.3-7).

Figure "Identifying surface fouling": In order to prevent fouling from having damaging effects, it is especially important to closely visually inspect parts before finishing processes that can cause damages (e.g. heat treatment and galvanic processes). Typical signs of fouling are shown in the bottom diagram. It may be necessary to consult the specialists in the specified information chain for evaluations and solutions.

Figure "Preventing fouling by masking": Covers are necessary in order to prevent fouling. However, they may themselves be a source of fouling.

“A”, masking with rubber covers during blasting to prevent deformation of filigreed sections. Metal tape is used during abrasive blasting. Wear products or residue from metal tape can cause damages in subsequent processes (Ills. 16.2.2.3-9 and 16.2.2.3-19).

“B”, covering the blade roots of turbine blades in order to prevent coating with a brittle diffusion coating, which would reduce LCF strength (Fig. "Aftertreatment cracks on Cr-coating"). Media can infiltrate under the cover. Any remaining powder can cause damage during subsequent heat treatment (Ill. 16.2.2.3-2).

“C”, covering against welding splash from electron beam, laser, and friction welding (Ills. 16.2.2.6-6 and 16.2.2.6-12). Splash can locally overheat and damage the surface (Fig. "Damages by splashes of liquid metal").

“D” in order to prevent splash from laser boring, hollow spaces in cooled turbine blades are masked with a wax filler (Ref. 16.2.2.3-7) that covers the opposing walls, and catches splash. In this case, it is possible that adhering splash may not have been completely removed, or may come loose later.

“E” masking hollow spaces. If worn or damaged covers allow blasting media to remain in hollow spaces, it can overheat hot parts or cause damages in the oil circulation system (bearings). If wear products are transferred to other areas, they can cause damages in subsequent processes (Ills. 16.2.2.3-9 and 16.2.2.3-19).

“F” covering during thermal spraying to prevent ricocheting particles from being sprayed (microsphere effect, Fig. "Properties of thermal spray coatings by production").

“G” rubber covers used to keep baths away from certain part zones. If the cover is infiltrated, etching baths can cause corrosion, and coatings can be undercut. If the covers remain on the parts for an extended period after the process, bath residue in gaps can cause damage.