Aeroengine Safety

Figure "HTC Classification"

Types of high-temperature corrosion relevant to engines, and their technical terms:

Oxidation: With the operating temperature ranges of modern engines and their typical structural materials, oxidation leads to creation of conversion coatings made from oxides of the base material, such as Al oxide and Cr oxide.
If these brittle oxides are erosively removed or damaged by strain (e.g. heat strain and volume changes, cracks and spalling; Fig. "Oxidation layer behaviour") then oxidation can progress until it unallowably weakens cross-sections and causes the affected part to fail.

Catastrophic oxidation is a type of oxidation with material loss that occurs due to the creation of liquid or volatile oxides. A typical example of a liquid oxide is vanadium pentoxide with a melting point of about 660 °C. Above 1000°C with high oxygen partial pressure, Cr oxides will largely evaporate. Al oxides, on the other hand, will not noticeably evaporate.

Coking: coking is an internal corrosion type that is based on the formation of carbides that result in a lack of those alloy components that create protective coatings. Damage through coking can occur around injection nozzles and the surrounding combustion chamber areas, especially underneath soot and coke deposits.

Metal Dusting: this special form of coking has been observed in Fe-, Ni- and Co-based alloys under constant coking conditions. It can cause heavy material removal. For this reason, this damage type is also called catastrophic coking (Ref. 5.4.5-1). In an intermediate stage, carbides form as internal corrosion products. Oxides, which are the end product of the damage process, typically fall off of the surface as dust.

Green Rot is also a type of coking which occurs, however, under alternating coking and oxidizing conditions (Ref. 5.4.5-1). The bonding of Cr prevents the creation of a protecting Cr oxide conversion coating, leading to heavy oxidation of Fe components. Extreme material removal was observed in one case in which, under low oxygen exchange, a Ni based alloy was bolted down with bolts made from Fe based material. This resulted in the Ni based alloy being seriously damaged through material removal with clear green discolorations.

Sulfidation (hot gas corrosion HGC, sulfidation): this oxidation process occurs under the effect of sulfur and, in oxidation-resistant materials, this process is several orders of magnitude faster than normal oxidation (Fig. "Sulfidation attack"). Sulfidation can be initiated and further accelerated by small amounts of silver (Ref. 5.4.5-2, Example "Silver coating"). For this reason, some manufacturers only allow the use of non-silver-coated bolts and nuts in certain hot parts.

Figure "Oxidation layer behaviour" (Ref. 5.4.5-1): Based on the mass changes, conclusions can be made regarding protecting or damaging properties of oxide coatings (top diagram). Mass increases that do not level off indicate progressive oxidation due to an oxygen-permeable (diffusion, porosity, and/or cracking) oxide layer.
The lower diagrams show models of the different damaging mechanisms of brittle oxide layers, where either volume increases induce compressive stress or volume losses induce tensile stress that exceeds the coating strength. As soon as crack initiation occurs in the oxide coating, there is a danger of substrate oxidation, which causes the oxide coating to break off, even if there are tension residual stresses present.

Figure "Damage mechanisms in hot parts" (Ref. 5.4.5-3): There are three primary damage mechanisms in hot parts, and they can occur in any combination:

• Solid and liquid deposits build up and cause geometric changes, affecting the aerodynamic performance of profiles or block cooling air bores. There is no shrinkage of supporting cross-sections. The built-up deposits cause a surface-dependent mass increase in the part.

• Solid and liquid particles cause erosive material removal and geometric changes combined with weakened supporting sections and changed aerodynamic properties. The part experiences a mass increase due to the “loading effect (Fig. "Unexpected behavior"). Progressive mass loss occurs after an incubation period.

• The reaction with gases and solid and liquid deposits (melts) causes strength losses without necessarily reducing the size of sections or changing their geometry. The reaction with surrounding media is accompanied by an almost linear reduction in mass due to a relatively slow loss of surface material.
  
  Combinations of these damage mechanisms can considerably increase the damage relative to a single effect. If, for example, a damaging reaction that creates erosion-sensitive surface layers is combined with erosive loads, then fresh metal surfaces are continually being created, which in turn can promote reactions. This can result in a self-increasing damage process.
  

Figure "Sulfidation attack": There are two different types of damage processes with sulfidation. Type I occurs above 800°C and Type II occurs between about 600°C to 800°C. These temperatures are not determined by the affected base material, but by the composition of the aggressive deposits. The primary damage mechanism is destruction of the protective oxide conversion coating, which lets in damaging salt melts. The components of damaging salt melts are sodium, vanadium, magnesium, calcium, and sulfur. Because engines burn relatively clean fuel, corrosion-inducing deposits generally come from contaminants in the intake air. Because sodium and sulfur have a special influence, engines that are in a marine atmosphere and/or being acted upon by dusts containing sulfur (such as dusts containing gypsum) are especially threatened. Experience has shown that silver, such as that on silver-plated bolts, can seriously accelerate the sulfidation process (Example "Silver coating"), whereby it probably acts as a catalyst. Silver deposits can evidently be created during standstill in the gas turbine through condensation water causing watery corrosion and acting on surface contaminations. These watery solutions are then transported to other part zones (e.g. flange lugs of the turbine disks). Evidently, corrosion-initiating silver again precipitates from these solutions. Experience has shown this corrosion process to be decisively dependent on the surrounding conditions. Pronounced damage of this type has only been reported in engines that were run in testing rigs in the vicinity of a chemical factory. Here, as well, it should be said that consciously introduced contaminants in the intake air (pollutant burning) must be specifically tested for potential effects such as the ones mentioned. The sulfidation damage mechanism can be explained as follows:

• The base material (e.g. Ni-based) is normally protected by a compact Cr₂O₃ coating, which sulfur cannot penetrate.

• However, different mechanisms, such as the chemical reaction of the oxide coating with salt melts, can allow sulfur to penetrate the coating.

• In this process, the metal ions in the salt melts change places with the Cr and “loosen” the Cr₂O₃ coating, allowing the sulfur to diffuse inward.

• Once the sulfur has entered into the base material, it prevents regeneration of the compact protecting Cr oxide coating, so that the salt melt is no longer necessary as a “door opener”.

• As a result, the base material is destroyed by accelerated oxidation.

Evidently even very small amounts of sulfur are enough for this damage process, since it only acts on a narrow zone at the transition to the base material and promotes the development of large amounts of Ni oxide. Even small amounts of nickel sulfide are sufficient for transporting the oxygen. Metallographic verification is correspondingly difficult. A non-destructive method for verifying sulfidation in exclusively externally accessible hollow structures (e.g. turbine rotor blades and stator vanes in their assembled state, Fig. "Oxidation symptoms on turbine blades") is magnetoscopic testing. It detects larger amounts of magnetic depleted base material which consists up to 90% from Ni and Co. Nonmagnetic is instead the dense oxide and the unaffected base material.

Figure "Oxidation symptoms on turbine blades": The inlet stator vanes of the high-pressure turbine and the blades of the first rotor stage are usually subject to extreme temperatures, despite intense cooling measures. If the normal oxidation-resistant coating is emaciated (“3”) or has suffered considerable thermal damage (e.g. spalling and rolling up), then an accelerated attack on the base material occurs, and can be seen in a rough and cracked surface (“orange peel effect”, arrows “1”) that appears dark and/or green, depending on the oxides that form. If the blade wall has been oxidized through (“2”), hot gas can enter into the cooling air ducts, causing cooling air losses and the danger of serious consequential damages not only to the directly affected part, but also to other parts (such as those that are connected to the same cooling air supply).

Figure "Internal sulfidation on turbine blades": Sulfidation is especially pronounced when the oxygen supply is not sufficient to allow a dense protective oxide layer to develop. This is evidently the case in hollow blades (weight reduction) that are closed on one end. In extreme cases, this results in “windows” opening up in the blades, i.e. both blade walls are broken through. This damage process is made more problematic by the fact that the extent of the damage can only be seen at a very late stage.
In blades with cooling air through-flow, there seem to be limited areas inside the blade where aggressive dusts tend to deposit (Ref. 5.4.5-7), and the part temperature is suitable for this phenomenon (Ref. 5.4.5-4). The temperature ranges necessary for sulfidation to occur determine the location of damages on the engine parts.

Figure "Perforation by sulfidation" (Ref. 5.4.5-8): This picture according to an photo demonstrates the extreme deterioration of a LP-turbine stator after some 1000 operation hours. The sulfidation attack (Fig. "Perforation by sulfidation") starts in the hollow vane profiles. If the stator has an “intermesh function” (Fig. "Intermeshing principle") the question for the remaining function arises.

Figure "Outer sulfidation on turbine blades": Sulfidation is limited to very specific temperature ranges (Fig. "Sulfidation attack") in which stable aggressive salt melts exist. For this reason, external sulfidation zones develop in very typical and part-specific zones. Aside from suitable temperature, these zones are also determined by a location suitable for deposits. These areas are usually on the pressure side of blades. In turbine blades, these zones are usually behind the bores of the film cooling (top left diagram); in uncooled blades with high edge temperatures, the zones are narrow fields parallel to the edges (top right diagram). Oxides created by the sulfidation are often emaciated due to erosion from the gas flow and particles it carries (bottom left detail), resulting in visible indentations.
If this is the case, it must be assumed that only a very short usable life span remains, relative to the operating time that has already passed.

Figure "Degradation during operation" and Figure "Damage mechanisms" (Ref. 5.4.5-5): The operating damages from atmospheric plasma-sprayed YSZ (yttrium-stabilized zirconium oxide) thermal barrier coatings (TBC) are primarily dependent on the surface temperature of the coating.
Damages in the left part of the diagram, especially those that lead to premature failures, were traced back to manufacturing problems. Curves “1”,”2“, and “3” show the failure behavior due to dust. These dust deposits include, for example, FeO+NiO from seal wear products and MgO+ CaO, Al₂O₃, SiO₂ from dusts ingested from outside the engine. Erosion is more pronounced at low operating temperatures.
During very long operating hours, effects occur that are based on changes in the coating structure:

• Phase changes despite the stabilizing yttriumA special type of phase reaction is the “mineralization”. It depends on a phase, which is mechanically stressed, or which is not in the equilibrium condition and migrates into the equilibrium condition.
  

• Sinter effects influencing the segmentation. At temperatures above 1093°C fresh sprayed thermal barrier coatings from zirconium oxide can sinter fast (Ref. 5.4.5-9). With this a markedly change of volume and material properties is triggered. Through sintering a shrinkage occurs, which leads to tensile stresses and releases cracks perpendicular to the surface (engl. („mudflat cracks”). The E-modulus (rigidity) of the sintered coating increases and it gets more brittle. Also the thermal conductivity rises, so that during constant cooling the component temperature rises. Examples for this property changes shows Fig. "Damage mechanisms".
  

• Oxidation of the undercoating (adhesion coating) triggers the so called “thermal grown oxide scale” (= TGO) between undercoating to the TBC (Fig. "Typical problems with thermal barrier layers", Ref. 5.4.5-11). The zirconium-oxide layer is penetrated to the undercoating by oxide. The oxidation, triggered by this process in many cases is the lifetime limiting effect. With increasing thickness of the TGO the adherence strength drops and spalling occurs together with a markedly rise of the temperature of the concerned hot part.
  

A special problem which gets more and more attention is the erosion of the thermal barrier coatings (TBC). Especially concerned are obviously thermal sprayed coatings. Concerned are break-outs of small particles from the surface. Besides a deterioration of the TBC itself, the life time of the aeroengine can be unacceptable concerned. The ceramic coating particles have an erosive effect at the blading, following in the gas stream. So for example the typical thin Al-diffusion coatings (ca.0,050 mm) can be removed in few hundred operation hours. With this they offer no more an oxidation protection. The result is an accelerated oxidation of the blading which makes a premature exchange necessary.

Figure "Typical problems with thermal barrier layers" (Refs. 5.4.5-6 and 5.4.5-6): The following are typical damage mechanisms in ceramic thermal barrier coatings made from ZrO₂ :

“A”: Erosion through particles and/or the gas flow. Damages to the high-pressure turbine segments are typical (housing-side seal surfaces across from the HPT rotor blade tips).

“B”: Melts of dust deposits can form in the combustion chamber and/or at the heavily heated thermal barrier coating (TBC) of the blading.For example such melts can be low melting silicates from ingested dust (Ref. 5.4.5-10). These can enter into the segmentation cracks (Fig. "Damage mechanisms") and create a bursting effect in the frozen state after cooling.
Salt melts may show this physical effect less pronounced. They shrink, for example vanadium-sulfate, during the freezing process in a manner, that dangerous high compressive stresses are not to be expected

“C”: Chemical reactions with deposits (e.g. fuel remnants). From tests can be identified, that compositions of the elements sodium, sulfur, vanadium and probably also lead and phosphor are primarily responsible for the hot gas corrosion of thermal barrier coatings. The aggressivity develops especially during oxidation of the fuel contaminations in the combustion in the combustion chamber. Thereby develop intense acid ore alkaline acting salt melts (Fig. "Sulfidation attack").

“D”: Delamination of the coating due to poor bonding (e.g. caused by manufacturing problems)

“E”: Oxidation of the contact surface, e.g. the undercoating, caused by oxygen ions being conducted through the hot ceramic coating. This failure mechanism has a relatively long incubation period (up to several 1000 operating hours), making it especially problematic. Also corrosive gases can penetrate the TBC. For example NACL vapour can let the oxidation layer faster grow at the adhesion layer. The so developing thicker oxide layer promotes the spalling.
Against corrosion ba salt melts, usually the zirconium-oxide of the TBC is more resistant as the highly oxidation resistant MCrAlY adhesion coatings. Observations bring close, that even thin ZrO₂ coating remains prevent an attack of the adhesion coating by salt melts.
Also solid organic oxides can deteriorate TBCs by acting acid.

“F”: Thermal stresses from manufacture and operation can cause spalling, especially at convex radii and edges. At convex surfaces like at the suction side of turbine blades the high compression stresses in the ceramiclayer act especially flaking. At concave surfaces the contrary is the case (Ref. 5.4.5-9). Simplified the relation is applied: The stress perpendicular to the surface curvature rises direct radius of the surface curvature (i.e., the more even the curvature, the more susceptible for spallings!), the coating thickness and the stress in the coating parallel to the surface.

“G”: Superficial crumbling after longer run times in a hot gas flow. These particles can considerably reduce the life spans of diffusion-coated hot parts (such as turbine blades).