Table of Contents
=== 220.127.116.11 Fundamentals of Corrosion ===
Figure "Types of watery corrosion": If corrosion-stressed parts are made from materials with differing corrosion sensitivity, then there is a special danger of crack corrosion in the edge areas (left diagram). This can be additionally promoted if elastic deformations result in micro-movements between contact surfaces/fretting, damaging insulating conversion coatings. Loosely contacting surfaces and weld gaps (Fig. "Corrosion risk in welded constructions") can suck in the electrolytes through capillary attraction and trap them, leading to corrosion of hollow structures from the inside out. This results in the extent of the damage remaining unnoticed until very late, sometimes even after sectional weakening has occurred and led to spontaneous part failure. The strength of the crack corrosion effect is also dependent on the surface area ratio in the corroding area between the corroding (small) and corrosion-resistant (large) surface.
Contaminants can create local cells. A local cell (second diagram from left) is a small-surface corrosive cell that has an effective electrode surface of fractions of a square millimeter. Local cells can be local material contamination or structural differences, or even protective coatings, deposits of foreign material, locally hardened or altered areas in the surface structure.
Contact corrosion can always be expected when parts made from different materials have electrical contact. The corrosion is generally stronger, the farther the positions of the materials (or their cells) are apart in the electrolytic series. This material combination then forms a galvanic cell with an anode (mainly anodic metal dissolution), cathode (little or no metal dissolution compared with the anode material), and electrolyte (third diagram from left, also see Fig. "Corrosion sensitivity of material combinations").
The metallic contact can be created by the contact between the surface, but it can also occur in welds and solders. However, it may also result from foreign metals smeared onto metal surfaces by repair or assembly tools.
A differential aeration cell (right diagram) is caused by an electrolyte droplet on a metal surface. Increased corrosive attack occurs underneath the drop, while the greater presence of oxygen around the periphery creates an immune area here. Hand sweat and condensation water corrosion on roller bearings are typical for this corrosion form, and have characteristic corrosion rings around each droplet.
Figure "Corrosion sensitivity of material combinations": Contact corrosion plays an important role in parts that are connected by positive fitting, friction (threaded connections, etc.), and material bonding (soldering/brazing etc.). Contact corrosion is related to cell action. If two metals are in metallic contact with one another through an electrolyte (usually a conducting liquid), then the dissolution of the less noble metal increases. Dissolution increases with the potential difference of both metals.
The increased use of carbon fiber-reinforced plastic materials must be mentioned in connection with contact corrosion. Carbon fibers behave like noble metals in their electrochemical properties and can, for example, cause serious corrosion if they come into contact with Al alloys. For this reason machined carbon fiber surfaces (exposed, cut carbon fibers) must be separated from corrosion sensitive materials by an intermediate insulating layer, or must only be connected to corrosion-resistant metals (such as Ti alloys).
Contact corrosion can be prevented through suitable material selection as shown in the diagram and/or through an electrically insulating separation of the metal surfaces. This is achieved through proper constructive design which avoids corrosion-promoting configurations (Fig. "Corrosion risk in welded constructions") which would lead to crack and/or contact corrosion. For example, especially corrosion-increasing configurations are those that include contact between parts made from metals with different potentials that result in direct metal contact. In contact corrosion, the potential difference between the pure contact metals is often not the deciding factor, but rather potential differences that, through so-called “practical series”, can be named as potential corrosives in practical media. Influences such as conversion coating formation, electrolyte composition, and electrolyte temperature are especially important.
Notes regarding the chart:
“1” Despite this, this metal combination is used in engine construction (even without protection).
“2” The parts should be lacquered.
“3” Is frequently used without protection in threaded connections in rotor and housings.
“4” Used in engine construction in threaded connections in rotors without any known problems.
“5” Dense coatings are a prerequisite for successful use.
“6” Danger of embrittlement through diffusion processes.
“7” Problematic with permeable coatings (damaged, cracks, porosity).
Figure "Corrosion due to cell action" (Ref. 5.4.1-2): Experience has shown that corrosion damage due to cell action is to be expected when one is “fixated” on another problem, such as erosion protection, and selects a coating based solely on this condition. For example, with an apparently corrosion-proof material such as 17/4 PH (17% Cr-steel type), ocean atmosphere can promote pitting corrosion due to cell action with erosion protection coatings. Merely the notch effect of the resulting corrosion pittings can considerably reduce the dynamic strength of the engine part.
Cell action can also occur at the edge of armor/hard facing made from electrochemically noble materials (such as WC in a Co matrix, top diagram), or in areas where the coating has been broken out, such as are typical after foreign object strikes on thin hard coatings (e.g. TiN; bottom diagram).
Figure "Failed erosion protection": Because the compressor blades made from an Al alloy in this engine type exhibited heavy erosion despite an organic coating, it was tried to increase the erosion resistance with a nickel coating. This diagram shows a typical blade after a salt blasting test in the laboratory. Extreme corrosion occurred on the blade, especially at the edges and artificially created FOD notches. Despite the excellent corrosion strength of the coating and acceptable corrosion resistance of the base material, this type of erosion protection was unsuitable due to cell action. The following factors especially promoted corrosion:
- “Nobler” coating material with high corrosion resistance on a less corrosion resistant base material.
- The coating had to be precipitated onto the metallic (i.e. unprotected) base material surface. This necessitated direct contact between the Ni coating and the Al alloy, which is very corrosion sensitive due to the lack of an oxide coating.
- An initially relatively large corrosion-resistant surface of the nickel coating compared with the smaller corroded base material surfaces under the coating damages. This poor surface ratio increases the corrosion effect.
- Foreign object impacts and erosion can locally damage the coating and make it permeable for electrolytes (sea water).
- Crack initiation in the nickel coating due to differences in heat strain relative to the base material.
- Sub-surface migration and delamination of the nickel coating.
Figure "Intergranular corrosion mechanism,5:54:541:5411:sensitization-of-stainless-steel.svg" (Ref. 5.4.1-3): Endangered materials are insufficiently stabilized, sensitizable CrNi steels that should no longer be used in modern engine construction, but may be used due to material selection errors or improperly controlled vendor parts (Fig. "Sensitization during operation"). “Sensitized” non-rusting steels are especially sensitive to corrosive attack along the grain boundaries. The sensitization is due to Cr depletion of the grain boundary area due to Cr carbide formation (see ). This is true both for intergranular corrosion and pitting corrosion at the grain boundaries. Sensitizing can occur in non-stabilized materials (without additional carbide formers such as Nb or Ta). The corrosion-sensitive structural state depends on the temperature and the time taken by the temperature introduction (top diagram). If the temperature is very high, such as is in a weld, then correspondingly short times (seconds) are sufficient for sensitization. If the temperatures are relatively low, such as operating temperatures (Example "Manufacturing errors"), then it may take several hundred hours for sensitizing to occur. These processes can cause long-term damage to engine parts that, as new parts, met all requirements with regard to intergranular corrosion attacks. It is also plausible that certain etching procedures that were successful with new parts could cause problems with run-in ones. For example, in an overhaul, parts with many operating hours can show unallowable etching due to a long-term structural change. An example of this is turbine blades made from an Ni-based cast alloy, which showed extreme intergranular corrosion after it was given an etching treatment in the course of an overhaul. The corrosion was so serious, that large grains fell out of the surface. Where this was not the case, crack initiation could not be safely detected by a penetration material test because the cracks were evidently filled corrosion products. The parts were noticed purely by chance and replaced. The inner damping of the blades had increased due to the cracks. When the blades were tapped, it resulted in an unusually dull sound. During heat treatment of sensitive materials at a middle temperature range, a span of minutes or hours is enough to cause sensitization. The damage risk can be minimized by observation of the sensitivity limits (bottom diagram).
Figure "Corrosion risk in welded constructions": The housings/casings of older engine generations, especially, are often welded constructions made from steels that are not corrosion-resistant. For these constructions, it is important that no outward-facing open gaps or hollow spaces area are created/remain; these are collecting areas for corrosive media and can not usually be sufficiently protected from corrosion. Sufficient corrosion protection can be achieved by, for example, welding on both sides. Threaded corrosion-sensitive contact surfaces (such as threads on Al and Mg cast parts) or plain bearings (such as the bearings of adjustable compressor stator vanes made from Cr steel in compressor housings made from Mg alloys; also see Fig. "Critical areas of a variable vane system") should be insulated from one another through non-conducting inserts such as synthetic materials, sealing compounds, or lacquers. These can both seal gaps and prevent metallic contacts.
Constructions should be designed in a way that unallowable damage to corrosion protection coatings can be avoided. This is true, for example, for contact surfaces between threaded parts with different stiffnesses which are subject to temporally changing loads (such as vibrations and heat strain) with relative movements.
5.4.1-1 A.Bäumel, “Überblick über Korrosionsschäden an metallischen Werkstoffen unter besonderer Berücksichtigung der nichtrostenden Stähle”, J.Grosch “Schadenskunde im Maschinenbau”, TAE Kontakt & Studium, Volume 308, Edition 1995, ISBN 3-8169-1202-8, page 250.
5.4.1-2 H.J.Kolkmann, “Gas Turbine Compressor Corrosion and Erosion in Western Europe”, AGARD-CP-558, proceedings of the conference “Erosion, Corrosion and Foreign Object Damage Effects in Gas Turbines”, Propulsion and Energetics Panel Symposium, Rotterdam, The Netherlands, 25-28 April 1994, Chapter 30, page 6.
5.4.1-3 ASME, Metals Handbook Ninth Edition, Volume 11“Failure Analysis and Prevention”, 1986, page 428.
5.4.1-4 P. Forchhammer, “Korrosionsschäden an metallischen Werkstoffen ohne mechanische Belastung”, contribution in “Systematische Beurteilung technischer Schadensfälle”, published by G. Lange, DGM Informationsgesellschaft Publishers, pages 263-289.
5.4.1-5 John Hickling, “Korrosionsschäden bei zusätzlicher mechanischer Beanspruchung”, contribution in “Systematische Beurteilung technischer Schadensfälle”, published by G. Lange, DGM Informationsgesellschaft Publishers, pages 291-311.