Dissimilar metal crevice corrosion

Long term tests of dissimilar metal crevice corrosion in filtered seawater Mater. Performance 33 (1994) 4, p. 51—54 [iiq Moretti, G. Quartarone, G. Tassan, A. Zingales, A... 

As noted in Table 3 [77], the results of multiple alloy tests in seawater are correlated to the Pitting Resistance Equivalence (PRE) number [47,48,72,77], which also correlates to alloy pterformance in FeCly. Anderson [78] and Streicher [79] used MCA in seawater tests to compare alloy performance. More recently, a simple, flat, plastic (specifically, polymethylmethacrylate, which is often referred to as "perspex ) washer has been successfully used to evaluate a series of alloys in seawater [77]. One application of the ASTM G 48 test has been in simulating leaking tube-to-tube sheet joints in seawater heat exchangers and condensers [87]. When certain highly corrosion-resistant alloys were paired in a dissimilar metal crevice (DMC) with alloys that would be expected to suffer crevice corrosion in the particular test solution, the more corrosion-resistant alloy was found to corrode due to the accelerating effects of the corrosion products from the less resistant alloy. The results of DMC tests in ferric chloride were confirmed by long-term DMC exposures in seawater [82],... 

Keams, J. R., Johnson, M. J., and Grubb, J. F., Accelerated Corrosion in Dissimilar Metal Crevices," Paper 228, NACE CORROSION/86 Conference, NACE, Houston. TX, 1986. 

A special form of crevice corrosion was observed in crevices between superferrite and austenitic standard steels in condenser pipes. Corrosion products, which lower the pH levels in the crevice, cause depassivation and increased corrosion on the ferritic steel. This form of corrosion, also known as dissimilar metal crevice (DMC) corrosion, does not occur with the combination of superferrites and superaustenites [115]. 

Copper alloys often show only weak crevice corrosion. This is especially the case if the copper alloy is coupled to a less noble alloy such as steel. The corrosion of the steel is stimulated by the galvanic effect caused by the coupling of dissimilar metals. Hence, the sacrificial corrosion of the steel protects the copper alloy (Fig. 2.9). See Chap. 16, Galvanic Corrosion. ... 

The formation of crevices between dissimilar metals should be avoided. Corrosion at such connections is generally more severe than either galvanic or crevice corrosion alone. Also, crevices between metals and certain types of plastics or elastomers may induce accelerated rates of combined crevice and chemical attack. Testing is recommended prior to establishing final design specifications. 

The US Bureau of Mines found the chemical and galvanic corrosion behaviour of both the TZM and Mo-30W alloy to be generally equal or superior to that of unalloyed molybdenum in many aqueous solutions of acids, bases and salts. Notable exceptions occurred in 6-1 % nitric acid where both alloys corroded appreciably faster than molybdenum. In mercuric chloride solutions the TZM alloy was susceptible to a type of crevice corrosion which was not due to differential aeration. The alloys were usually not adversely affected by contact with dissimilar metals in galvanic couple experiments, but the dissimilar metals sometimes corroded galvanically. Both alloys were resistant to synthetic sea water spray at 60°C. 

Finally, mechanical joints, e.g. nuts, bolts, rivets etc., are still important joining methods for which attention must be given to compatibility to avoid dissimilar metal corrosion problems and crevice corrosion " . ... 

Design (avoidance of dissimilar metals, galvanic couples, improper materials, high fluid velocities in inappropriate places, caulking or seal welding of areas prone to crevice corrosion, roof design, etc.)... 

Over the past 40 years, automobile engineers have improved the design to reduce the extent of corrosion. The design improvements consisted of removing crevices and locations where salt and soil can accumulate. Dissimilar metal contacts were removed. The number of nose over hoods, hood louvers, tuck-under areas, and other design features that promote chipping and corrosion have been reduced. 

Recently, heavy coatings that act as sealants for crevices have been developed for the aircraft and aerospace industries. These coatings contain proprietary inhibitor formulations that are especially effective in minimizing corrosion associated with dissimilar metal fasteners (Moiseeva 2005). 

Factors affecting localized corrosion of nickel-base alloys are chloride concentration, pH, temperature, crevice geometry (depth and tightness), and crevice former material (non-metallic or similar or dissimilar metal, metal surface condition, area ratio of exposed to shielded metal). A wide range of results can be obtained as these factors are varied. 

Service life can also be affected by galvanic contact with a dissimilar metal. The less resistant material tends to be dissolved and may experience general corrosion, pitting/crevice corrosion, or SCC. Hydrogen may be liberated at the more resistant metal, making hydrogen embrittlement an issue if the material is susceptible. Stray currents, e.g., from a DC power source, may have the same effect as dissimilar metal contact. 

Crevice corrosion occurs in cracks or crevices formed between mating surfaces of metal assemblies, and usually takes the form of pitting or etched patches. Both surfaces may be of the same metal or of dissimilar metals, or one smface may be a nonmetal as shown in Fig. 6.20. It can also occur imder scale and surface deposits and under loose fitting... 

Corrosion control in recirculating water systems is always complicated by the presence of dissimilar metals in contact, crevices, and areas where an oxygen defidenqt exists (differential aeration), e.g. welds and joints. Unfortunately, protective paint schemes almost always break down in these areas and localized corrosion follows. 

Formation of crevices between dissimilar metals shall be avoided corrosion of such connections is more severe than either galvanic corrosion or crevice corrosion on their own (see Figure 9.4). 

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