Table of Contents

5.4 Corrosion

- 5.4.1 Corrosion without Mechanical Loads
- 5.4.2 Stress Corrosion Cracking (SCC)
- 5.4.3 Corrosion Fatigue
- 5.4.4 Hydrogen Embrittlement
- 5.4.5 High-Temperature Corrosion

The increased use of titanium and nickel alloys reduces corrosion susceptibility in modern engines. Older engine types with compressor blades made from Cr steels and aluminum alloys, rotor disks made from steel, and housings/casings made from magnesium alloys or low-alloy steels, on the other hand, have a higher risk of corrosion.
Even in modern engines, corrosion-sensitive steels are used in casings and compressor blades in those areas where matched thermal expansions are necessary to maintain clearances. For example, in high-pressure compressors, disks made from corrosion-sensitive Cr-steels with “balancing” expansion behavior are used between titanium-alloy rotor disks and disks made from Ni-alloys. Corrosion damages additionally occur on special abradable coatings in the housing of the front compressor area. Extra attention is also given on hot gas corrosion in hot parts. An overview of the corrosion behavior of typical material families in engines is given in the table in section
There are various types of corrosion with different damage mechanisms (Fig. "Behavior of a corrosions system").

Most corrosive media are brought into the engine along with the intake air. Upon entering the engine they can cause damage during operation. In the case of water corrosion, however, the standstill times of the engines are the deciding factor. Corrosion begins when condensation water is present and dissolves contaminants in the engine. Uncovered engines on aircraft that stand out in the open during the daytime and nighttime for longer periods experience air exchanges due to temperature variations and wind. This allows corrosive media (such as in ocean atmospheres) and/or humidity to enter the engine. When condensation water is created, the conditions for standstill corrosion are met. For this reason, experience has shown that engines of aircraft with longer standstill times in areas with ocean atmosphere are subject to the greatest corrosion stress.

The above table shows that engines in military aircraft are exposed to much worse operating conditions with regard to corrosion than are engines in civilian aircraft. For example, an engine in a commercial passenger aircraft had less corrosion after 12,000 (!) flight hours than another engine of the same type in a fighter-bomber had after only about 500 flight hours.

Corrosion can also occur due to improper storage of engines (such as in containers with condensation water formation) or individual components that are insufficiently conserved in non-climatized storage areas (such as preassembled transmissions).

If one includes etching damage, especially, an additional possibility for corrosion damage is during the production or repair of engine parts if, for example, unsuitable cleaning or etching processes are used. Quality controls are not always sufficient to prevent the installation of parts damaged in this way.

During mounting and maintenance there is a danger that unsuitable auxiliary materials (washing media, lubricants, cleaning materials) may be used.

Corrosive media:
There are many corrosive influences that affect engine compressors. These influences can be of very different types and have various increasing effects on one another. For example, one might consider an erosive influence that removes protective oxide layers or lacquers (abrasive cleaners, etc.), or perhaps a process in which corrosive deposits build up on certain coatings that are not particularly corrosive on their own (soot, oil remnants, etc.).

Typical corrosive media in the intake air:

  • Sea salt in a watery solution in various concentrations or even as dry deposits. These contain primarily NaCl, MgCl2, MgSO4, and CaSO4.
  • Industrial atmospheres, i.e. deposits created by air contaminants from industrial processes. Especially strong corrosive attack can be expected from acidic watery solutions, such as sulfuric acid resulting from dissolved SO2 and/or soot deposits.
  • Soot deposits due to recirculation of the engine exhaust gases and oil remnants.
  • Washing material remnants from compressor washing.
  • Remnants from mechanical compressor cleaning procedures.
  • Dust deposits: The corrosive effect may depend on the different compositions of the ground where the dust originated (e.g. high content of iron oxides or sulfur compounds such as gypsum). An especially corrosive atmosphere is that in hot dry regions such as deserts and steppes which are irrigated with ground water with a high saline content. The salt deposits remaining after the water evaporates are thrown up into the air with the dust, then settling on the engine components and corrosively attacking them.
  • Exhaust gases from munitions (cannons, rockets)

Typical corrosive media from manufacture and overhaul:

  • Remnants and reactive coatings from cleaning baths (overhaul) such as chlorides on titanium parts or acid remnants in the cooling air ducts of turbine blades.
  • Insufficiently reacted coating components (such as in multi-component coatings)
  • Deposits and wear products from manufacturing processes such as grinding dust and brush wear products. Tool wear products from cutting and non-cutting forming processes.Wear products from assembly tools.
  • Contaminants from abrasive and hardened peening materials. These wear products can be caused by the peening particles themselves, such as micro-bonding or adhesion to the surface, as well as by peening material contaminants such as lead (from a protective lead cover).
  • Unsuitable lubricants (e.g.on bolts and adjustable components) and sealing materials. These can have corrosive effects without thermally decomposing (for example, sea salt acting on lubricants containing MoS2 on joints made from steel parts), or only after decomposing (for example, bolts in hot parts under the influence of high temperatures).
  • Unsuitable conservation oils or use of wrong oil type (for example, cutting oil instead of conservation oil).
  • Writing on parts with unsuitable felt-tip or oil markers.

Typical corrosive media from inside the engine itself:

  • Construction related: Unsuitable material combinations (for example, cell action related to silver-plated parts in the hot part area), use of corrosion-sensitive materials (see table in section, or material properties (for examples, Cr steels with excessive hardness with regard to SCC).
  • Operation-specific: deposits and decomposition products from leaking bearing oil.
  • Decomposition of oil with additives: Transmission casings made from Mg alloys can be attacked by acids in the oil. Non-protective oxide layers can promote sulfidation in nickel alloys.
  • Corrosion products from other components in the form of deposits in the downwind engine areas (for example, in the cooling air ducts of hot parts).

Figure "Corrosion-stressed components":
Compressor rotor and stator blades “K1”: Pitting corrosion in blades made from Cr steels. Corrosion in soldered blades can be increased by cell action with the nobler solder (gold, silver, copper; Fig. "Corrosion in the area of the compressor"). In blades made from Ti alloys there is a potential danger of hot salt corrosion at temperatures above 450 °C (Fig. "Design of experiment").

Compressor disks “K2”: Steel compressor disks such as are found in older engine types can exhibit pitting corrosion (Fig. "Corrosion as cause of a disk burst"). In some modern engine single disks made from steel (requiring corrosion protection) with a suitable expansion coefficient are used between disks made from Ti alloys and disks made from Ni alloys in the hot rear engine area.

Compressor housing “K3”: Compressor housings of older engine types made from Mg alloys are susceptible to corrosion through ocean atmosphere and humidity (Fig. "Corrosive attack on Mg alloy"). In these engine types, rear compressor housings and combustion chamber casings are often made from low- and high-alloy steels that require corrosion protection.

Rub coatings and abradables “K4” and “K5”: Components of these coatings, such as Al powder used as a filler in a polyester matrix, can be seriously damaged by corrosion (Fig. "Corrosion of rubbing coatings during marine operation"), even causing them to break out or blister.

Compressor blade adjuster “K6”: Components of blade adjustment systems such as bearings, connectors, axles, and cables consist primarily of steels that can suffer corrosion damage (Figs. "Critical areas of a variable vane system", "Stator vane adjuster", Vulnerable components" and "Failure of variable vane adjuster")

Roller bearing “K7”: In roller bearings there is corrosion danger due to condensation water during standstill. Before installation, bearings, especially disassemblable ones, must be inspected for corrosion due to hand sweat.

Gearbox housing “K8”: The housings of the auxiliary gearboxes are usually made from Al alloys or Mg alloys. Housings made from Mg alloys, especially, are prone to corrosive attack from the outside due to ocean atmosphere and from the inside due to aged oils (acidic).

Multiple splining on shafts “K9”: These parts, which are usually made from low alloy steels, are susceptible to friction corrosion when affected by moisture in combination with micro-movements of the contact surfaces that are typical during operation.

Threaded connections and clamping bolts “K10”: When these components are typically made from high-strength steels, improper heat-treating creates the possibility of corrosion-induced crack initiation and pitting corrosion followed by fatigue (Figs. "Corrosion as cause of a disk burst" and "Fracture of corroded clamping bolt").
In threaded connections and bearing surfaces made from nickel alloys, there is a danger of damage due to decomposing lubricants that contain sulfur and/or sulfur from the fuel, which increase the corrosion process and often act in connection with silver coatings (Example "Silver coating", Ref. 5.4.5-2).

Hot parts“K11”: Sulfidation and hot gas corrosion under the influence of contaminants in the hot gas. Under certain conditions (temperature, composition of deposits) all materials with a Ni- or Ci-basis are at risk (Chapter 5.4.5).

Hot parts in contact with silver “K12”: Silver on coated parts such as bolts and nuts can dissolve when attacked by acidic electrolytes (for example, due to condensation water that has been contaminated by an industrial atmosphere) and build up deposits on disks and blades. There it can cause dangerous pitting corrosion at operating temperatures.
Sulfidation can be considerably increased by silver in the form of a wear product, coating material, or splashed melt.

Hinge bearing “K14”: These bearings are usually made from Martensite steels and are prone to rust in ocean environments in case of insufficient lubrication or corrosion protection. Experience has shown that in these cases grease containing MoS2 can evidently increase the corrosion between glide surfaces and lead to jamming. In this area, normal engine oil still proves to be effective.

Figure "Corrosion media ingestion": The potential corrosion media are already at the beginning of this chapter discussed in detail. Therefore in the following the presentation will be limited at the kind of ingestion into the aeroengines.

Ingestion during flight: Also in high altitudes the danger exists, that aeroengines will intake deteriorating media. Primarily concerned is volcano ash (Fig. "Danger of volcanoes") and gases (e.g., rich in sulfur). Also sand storms can transport fine dust in high altitudes. The deterioration potential depends especially from the composition of the particles. So for example, a high content of gypsum (sulfates) intensifies the triggering of sulfidation in hot parts (Fig. "Internal sulfidation on turbine blades" and Fig. "Perforation by sulfidation").

In low altitudes during start and landing as well as flight missions of military airplanes like fighters and for surveillance (e.g., for the control of submarines) the ingestion of the corrosion medium takes place by intake. Further the danger of corrosion comes from ingested residues of combustion (gases, particles) during firing of weapons like missiles and machine cannons (Ill During flights in low heights with high speed in the aeroengines of fighter aircraft occur especially high operation temperatures. So aging and decomposition processes at materials (e.g., plastics) and auxiliaries like oil can be intensified. Aging products can act for example with condensate as corrosion medium and attack corrosion susceptible materials like magnesium alloys (gear boxes or aluminium filled rub in coatings, Fig. "Typically affected components") and/or seals from elastomers.

On the ground during running aeroengines: Here the special danger exists, that dusts, water and foreign objects are sucked in (Fig. "Vortex I"). The intaken amount can increase during landing by activation of a thrust reverser (Fig. "Thrust reverser").
The experience shows that during test and certification runs on the ground an increased danger of ingesting corrosive media exists. To these belong e.g., deteriorating industrial gases, especially from chemical production processes, cleaning/washing, painting and electroplating. So, in one case pitting corrosion at turbine disks of the rear stages made from a Ni-alloy was identified. The attack could be traced back to ingested exhaust gases from an electroplating plant, together with condensate during stand still over night.
A special situation arises from outdoor so called field test rigs in the military application. Here an increased potential danger exists, that deteriorating media from the environment are ingested (Fig. "Air pollution"). To these belong media from agriculture like fertilizer and pesticide.
A not so seldom situation occurs, if fire extinguisher (Fig. "Fire extinguishing media") gets into aeroengines and there, especial in the hot part, discharges its typical aggressive components.

Helicopter engines are especially endangered by dispersed dust. This can be increased in two manners. By erosion effect arise fresh reactive surfaces which are especially prone after the shut down of the aeroengine. Also a recirculation of the exhaust gases with the ingestion of soot is especially likely at helicopters as well as vertical take-off and landing airplanes.

At the ground parked airplanes: It is this situation, at which usually the major part of the corrosion deterioration (idle corrosion) arises. Especially when immediate before, erosion produced reactive surfaces. For this reason special aeroengines from military application with long stand still periods and not necessarily long operation times are especially concerned. Also during stand still corrosive media can enter. Are the aeroengines not sufficient covered, the danger of a pollutant entry exists by wind and air exchange. Temperature changes (day/night) lead to corresponding change of the air volume in the aeroengine and with this alternating inflow and outflow.

Figure "Behavior of a corrosions system": Three main factors, i.e. corrosive media, materials, and mechanical loads, determine the behavior of a corrosion system. The depiction with the overlapping circles is based on Ref. 5.4.1-5. The three main influences are themselves affected by many different conditions (top diagram). This shows the complexity of corrosion processes.
The bottom three diagrams show the conditions for three typical crack-initiating corrosion types (black areas). As shown, the occurrence of stress corrosion cracking (SCC) requires the combination of all three main influences. If one of these influences is missing, such as sufficient tensile stress levels or stress concentration, then SCC can not occur. With corrosion fatigue, a specific medium and sufficient (dynamic) loads are necessary, but a special material state of the affected material (such as 13% Cr steels in the typical state during operation) is not a prerequisite.
In order for intergranular corrosion to occur, a sensitive material and a specific medium are necessary, but there is no specific minimum load level necessary for this type of corrosive attack.

Figure "Local corrosion types" (Ref. 5.4.1-1): The illustration is based on the mentioned literature. Locally acting corrosion types form notches, increasing the loads on the part. This leads especially to a noticeable decrease in the dynamic strength of the part.

Pitting corrosion: This type of corrosion occurs when Cl ions break through the inert surface layer and activate the base material. These areas of weakened inert surface layers are created by surface contaminants such as smeared foreign metal or inhomogeneities in the material. Pitting corrosion (top left) occurs especially in Cr steels (such as in the compressor blading of older engine types), if there is no sufficient (cathodic) corrosion protection.
A unique problem is posed by thin oxidation layers (tarnishing) due to heat development during manufacturing processes or operation. In CrNi steels these oxide layers have a high chromium content which was taken from the surrounding base material, increasing its sensitivity to corrosion. The structure of these oxide layers has micro-cracks and allows the Cl-rich electrolytes to act on the corrosion-sensitive base material, resulting in pitting corrosion.

Stress Corrosion Cracking: This type of corrosion (top center diagram) can be either transgranular (austenitic Cr/NI steels of type 18 9) or intergranular (e.g. typical 13% Cr steels). The requirements for SCC are described in Fig. "Behavior of a corrosions system". Active material dissolving occurs in the base of the crack. In many cases typical signs of the influence of hydrogen can be seen at the micro-level (Fig. "Effects of hydrogen on the structure"). The hydrogen is created during the chemical dissolving process and enters into the opened area at the tip of the crack, which is not protected by an inert coating, in a process in accordance with the illustrated model. If the crack growth is promoted by the influence of the hydrogen, it can lead to apparently spontaneous fractures. In general, the SCC sensitivity of a material increases along with its strength (hardness).

Intergranular Corrosion: This corrosive attack of grain boundaries (top right diagram) is due to the sensitivity of the grain boundary region. In this zone, the alloy component necessary for corrosion resistance is usually not sufficiently present in its corrosion resistant form. In the grain boundary area of sensitized CrNi steels (usually CrNi 189), it is typical for Cr to be bound in the form of carbides and no longer able to form an inert protective layer. This type of sensitization can occur at high operating temperatures (Ref., Figs. "Sensitization of stainless steel" and "Coating damage I"), as well as during manufacture with improper heat treatment or in areas affected by heat from a weld. The prerequisite for sensitization is a composition-dependent sensitivity of the material. The addition of small amounts of strong carbide forming materials such as Ti, Nb, or Ta can ensure that enough Cr remains free to provide corrosion protection.

Crack corrosion (bottom left diagram): In order for a sufficient protective passive layer to form, there must be sufficient oxygen in the surrounding atmosphere. This is usually not given inside cracks. For this reason, the corrosive medium (such as Cl ions) can locally break through the weakened passive layer (phase 1). At first pitting-corrosion-like pittings are formed. These expand (phase 2) after the resulting corrosion products further activate the crack area.
Crack corrosion is promoted by cell action, i.e. contact between surfaces that are located considerably far apart in the electrolytic series (see Fig. "Corrosion sensitivity of material combinations").

Separation corrosion: When an alloy melt hardens, a partial segregation, i.e. concentration of alloy components occurs, at least on the micro level. With forged parts, the fine granularity and the deformation allow corrosion to occur in many micro-areas in a larger zone. Endangered parts include, for example, milled products in which a separation zone occurs at the surface (Fig. "Vulnerable components"). In cast parts and also in welds, the zones of uneven alloy composition can be much larger.

Figure "Corrosion near weld seams": Often, several different types of corrosion can be observed around weld seams. For this reason, this area is especially suitable for describing common corrosion mechanisms.

The top three diagrams show corrosion forms that can occur even without noticeable mechanical loads. Surface corrosion occurs in areas that are sensitive to the acting corrosive media. However, the affected surface does not show any pronounced sensitivity of individual components at the micro-level, resulting in largely even wear. This can occur either in the seam itself (top) or the heat-influenced zone.
If there is locally concentrated corrosive stress, such as due to cell action around contaminants or structural inhomogeneities, it can result in a corrosive attack that causes pitting (pitting corrosion).
In crack-like corrosion forms, the crack characteristics largely depend on the mechanical tension in the corrosion area (Fig. "Crack growth mechanism").

Intergranular corrosion occurs along corrosion sensitive grain boundaries. This sensitivity can be caused by a “sensitization process”, which requires a specific temperature range. Intergranular corrosion can be promoted by tension stresses.

Stress Corrosion Cracking (SCC, Chapter 5.4.2) requires not only a material or material state that is sensitive to a specific corrosive media, but also sufficiently high tensile loading.

Corrosion Fatigue (Chapter 5.4.3) occurs as a combination of a corrosive attack with vibrating loads. Fractures and cracks usually originate in notches and corrosion pittings.

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