Table of Contents
3.1.2.3 Corrosion and erosion
Almost every car owner feared at least in former times that corrosion devaluates his car. Especially aggressive in winter acts the road salt if the car was standing for a longer time. Today with the zinc coating (galvanization), now a technical standard, the fear of former generations is hardly to understand. If rost holes were visible from the outside it was rather too late. It looks very similar in the gas turbine technology. Here also salt is the main harmful factor. In this case ingested with the air. Gas turbines with longer standstill time form during temperature changes condensation water. So they are corrosion loaded similar to a car. The compressor blading of elder engine types consist in 13% Cr-Steel. As polished, as blade of our fine household knifes, rust appears only at unsufficient care, e.g., it lies for a longer time in the sink. The compressor blades of a gas turbine are roughened by erosion. This enables a dangerous form of rust, the so called pitting corrosion. Those pittings dangerously decrease the fatigue strength up to blade fractures. Hence corrosion sensitive steel parts of a gas turbine are coated today with an anorganic coating filled with aluminium powder. This acts like the zinc coating on the car body by so called „catalytic protection“.
Corrosion occurs in the compressors in different forms. A material specific corrosion medium ingested with the inlet air is required in most cases. As the compressor emits large amounts of air, even small portions of corrosive acting materials in the air, in an enriched condition, e.g., as deposits on the wings, produce corrosion failures.
Corrosion, like pitting corrosion ( "Ill. 3.1.2.3-1"), through aqueous media, originates during static times of engines showing corrosion sensitive compressor materials, like blades out of Cr-steels or Al alloys, casings out of Mg alloys or abradables with Al filling. Especially risky are places with a marine atmosphere that cause these salt deposits to act hygroscopically, inducing aqueous electrolytes. Hence, it is to be remembered: the more static times an engine has, the greater the corrosion danger. Pitting corrosion is a local corrosion frequently penetrating tenths of millimeters into the material, supporting fatigue fractures through the notch effect, giving rise to the danger of extreme secondary failures.
Inorganic paints with Al content have proved to be optimal corrosion protection for steels in the entire compressor . These paints offer an anodic corrosion protection, (smaller, unprotected base material surfaces, e.g., in the region of scratches and scores, are protected through the surrounding), next to an extremely high permissible temperature of over 500 °C, as well as a certain erosion resistance.
Corrosion also appears during operation, not generally related to pitting corrosion but to types of corrosion that, together with a material specific corrosion medium, presuppose static and/or dynamic mechanical loads of sufficient size. In the front, relatively cold compressor region, at least, there is the possibility of the failure originating through moisture, even during operation. This is especially valid for corrosion fatigue ( "Ill. 3.1.2.3-1"), by which, during dynamic load, the corrosion medium is effective. According to experience, the frequently used blade materials from type 13% Cr-steel are highly susceptible to this, often linked to an already present pitting corrosion attack.
Even apparently corrosion insensitive materials like Ti alloys are endangered. One knows, at least from specimen trial attempts, that by sufficient tensile load and operation temperatures above abour 450 °C dry salt deposits can initiate strong crack formation. From failures in the cleaning area (cleaning in ‘tri’ or ‘per’) one is aware that stress corrosion cracking (SCC) can arise, if halogen (especially chlorine) is effective. Thus, the addition of Cl containing solutions in the inlet air flow is especially problematic.
Abradables in casings in the front compressor minimize tip clearance and are frequently formed out of polyester resins mixed with Al powder. These coatings, produced in a thermal spray procedure, are sensitive against selective corrosion of Al particles with crack and blister formation as well as subsequent spalling ( "Ill. 3.1.2.3-1").
Components in the air flow with synthetic paints, especially Al blades of older types of gas turbines are tried to protect with resin coatings aganst erosion and corrosion. Unfortunately those protections are unsufficient. The blades can be irreparably damaged on the grounds of a heavy drop of fatigue strength through a combination of erosion which destroys the coatings and corrosion that creeps beneath the paint and produces pitting corrosion.
Rotor intermediate rings, so called spacers, are often equipped with hard, ceramic, light porous coatings, (e.g., Al2O3 or ZrO2 ). Should this base material be insufficiently corrosion resistant, (steels) these coatings can corrode below coating flakes and extensive secondary failures at the blades.
Erosion in the compressor Compressor
Compressors without preceding effective filters have a potential erosion risk. Contains the filtered intake air only 1 ppm (percentage of weight1 to one million) dust a gas turbine sucks daily per 10 MW power output about 4 kg dust (Lit 3.1.2.3-1). Erosion is normally caused through abrasive dusts and particles brought in with the air flow. Already after short stretches, the dusts are centrifuged outwards, accumulate at the casing walls and the rotor blade tips. These component regions are correspondingly strongly stressed. Soft abradables (e.g. nickel, graphite, spray coatings) on the casing interior walls are damaged through the effects of this erosion and they in turn cause abrasive particles. Typical for the erosion on the blades is the weakening of the guide vane roots ( "Ill. 3.1.2.3-3"), whereby the fatigue strength drops to the first flexural mode. Erosion follows in the area of the rotor blade tips. This leads to a profile change with corresponding chord shortening ( "Ill. 3.1.2.3-3") and an enlargement of the tip clearances. Not seldom, the blade shows a so called „hooking“ (hook type shape of the leading edge near the blade tips ) in extreme erosion. Noticeable erosion in the rotor blade tip area has an unfavorable effect on the degree of efficiency.
A distinct roughness of the aero dynamic effective blade surfaces and deformations of the edges in the micro area ( "Ill. 3.1.2.3-3") are to be expected already during erosion load, which still do not lead to macroscopic, conspicuous, geometrical alterations. These are not enough to influence the entire engine markedly. Because these damages occur slowly over a longer period of time, higher than expected operation costs occur, at first unnoticeably (fuel consumption, hot part failures through high temperatures).
"Illustration 3.1.2.3-1": The corrosion in gas turbines occurs during standstill under the influence of condensation, forming aggressive electrolytes along with the deposits. In many compressors of gas turbines in industrial use, corrosion sensitive materials are used for blades (e.g., from 13% Cr-steels ) and casings (e.g., those made out of light metal alloys based on Al or Mg, "Ill. 3.6.1-5").These are usually protected with effective paint, respectively, coating systems. Corrosion resistant Ti and Ni base alloys are used, in a large measure, in aeroengines.
Only seldom, e.g., at high operation temperatures near the compressor end, diffusion coatings (chromalized) are used. They offer like galvanic chromium and nickel coatings only a corrosion protection as long as they seal the base material against the corrodent. Elsewise they can form a galvanic cell with the ignobly base material and so even support corrosion. This danger especially exists when the protective coating is damaged by fretting, abrasion or erosion.
Not only can the named metallic materials be attacked through aqueous corrosion, abradables ( "Ill. 3.1.2.4-1") are also corrosion sensitive due to their composition and frequently porous structure. Al powder filled polyester spray coatings (detail „B“) have shown themselves to be especially vulnerable. In blades, pit like attacks, (pitting corrosion ) are particularly hazardous on account of their notch effect (detail „A“), as a likely origin for fatigue fractures. Component zones with different materials that create possible element formation through metallic contact are especially corrosion sensitive. Typical for this condition are brazed areas (detail „C“) at built compressor blades from Cr steel with Ag or Cu- solder
Avoidance of corrosion: Avoid corrosion triggering and and supporting conditions. To those belong:
- Condensation by environment conditions or drying processes (warm air) during stand still.
- Remove corrosive deposits, e.g., by washing of the compressor ( "Ill. 4.2-1.2").
- Use of corrosion insensitive materials (e.g., titanium alloys) or with an insensitive structure. As far as possible corrosion resistent alloys on Titanium or nickel basis that are used in aero engines are mostly also used in their derivates.
- Otherwise use coating systems that have a cathode protective effect. This includes inorganic paints with Al powder filling in different versions (e.g., sealed and/or hard worked through glass bead peening).
- Avoid corrosion critical coatings. This can concern the coating itself but also the formation of a galvanic cell with the base material.
"Illustration 3.1.2.3-2": The diagram shows the conditions in front of the compressor (E) and at some front compressor stages up to about 3 bars end pressure. Moisture in this compressor region during operation (grey area) can form sticky deposits together with salt and dust (Lit. 3.1.2.2- 4). These deposits enlarge the roughness of the blade surfaces and cause fouling. A further problem is aqueous corrosion on sensitive materials . Concerned are blades and discs from 13% Cr-steels as well as casings out of light metals or low alloyed steels (see "Ill. 3.1.2.3-1"). This corrosion is not only effective during static but also during operation, supporting fatigue failures due to corrosion fatigue. Even the normally tolerated vibrations during operation ( "Ill. 3.1.2.4-4") of the compressor blades and vanes can cause fatigue cracks to emanate from corrosion pits. Corrosion fatigue then accelerates the crack propagation.
"Illustration 3.1.2.3-3": Erosion in the compressor originates in front, through particles in the inlet air. At the rear, noticeable erosion through abrasion of abradables in the casing or spalling of these coatings can also take place. The air flow itself can, when a damage of the coatings is already present or is simultaneously effective, (e.g., aging through oxidation of nickel/graphite spray coatings in the rear compressor or corrosion of Al powder filled ( "Ill. 3.1.2.3-1") synthetic spray coatings in the front compressor release particles and entrain them, similar to a sandstorm on the ground. Through the air flow, particles are quickly centrifuged towards the outside in the swirl, so that a typical erosion picture is produced on the blades: the rotor blades are eroded at the tips, the stator vanes near the root. This produces a higher fatigue sensitivity of the guide vanes, because of a cross section reduction of the blade profile, resulting in the most frequent occurrence of vibration in the first bending mode. Through this form of vibration, the weakened zone is especially highly stressed. Additionally, the bending frequency of the blade drops, as a consequence of the erosion influenced cross section reduction. The opposite takes place with regard to the rotor blade. Through the erosion at the tips, the blade mass in this area decreases the first natural bending frequency increases. A degradation of the blade does not follow. The erosion does not only change the macro geometry of the profile, it roughens the pressure side of the blade. Suction side erosion through reflected particles is clearly less than on the pressure side. Its appearance, would have an especially strong influence on the flow. The deformation of the leading edge in the micro area (detail), the tip clearance enlargement as a consequence of the erosion of the abradables and a macroscopic alteration of the blade profile, leads to a deterioration of the compressor efficiency stimulating consequences described in chapter 3.1.1. The conditions of the stages behind (higher pressure and high speed) lead to a thinner boundary film thickness. Therefore, even clearly slighter roughness will be effective on the flow, compared to the front compressor area.
The erosion of the abradables can contribute to the blocking of the hot parts ( "Ill. 3.3-12") resulting in their premature breakdown. In order to maintain a possibly clean cooling air, these are removed from the hub of the compressor rotor where the particles are centrifuged towards the outside. Engines that have an unfavorable removal zone are potentially at risk for such secondary failures.
Sucked in particles or such of eroded abradables can block the cooling holes of the hot parts ( "Ill. 3.3-12"). This can add to a premature failure and so strongly raise the operation respectively repair costs. To get a clean as possible cooling air it is taken near the hub of the compressor rotor. From this area the particles are widely centrifugalized. Engines with an inappropriate extraction zone are potentially more endangered by the blocking of the hot part cooling and its consequences.
Literatur of chapter 3.1.2.3
3.1.2.3-1 R. Swanekamp, „Monitoring and maintaining advanced gas turbines“, Zeitschrift „Power“ March/April 2001, Page 55-74.