Galvanic crevice corrosion

If there is a crevice on a component made of a material liable to crevice corrosion, and this component is connected to a more noble material with free surfaces, crevice corrosion may be intensified strongly. Such a case is a couple of an aluminium component (with a crevice) and a steel plate in water containing some chloride. The corrosion form can be called galvanic crevice corrosion. The crevice corrosion rate will be particularly high if the more noble metal acts as an efficient cathode in the given environment. The explanation is the same as for ordinary galvanic corrosion. 

Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

Potential differences leading to galvanic-type cells can also be set up on a single metal by differences in temperature, velocity, or concentration (see subsection Crevice Corrosion ). 

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. ... 

Wastage was caused by crevice corrosion, accelerated by the difference in tube and tube sheet metallurgies. The brass tube, being more noble, was cathodically protected by corrosion of the surrounding mild steel tube sheet. However, the galvanic effect was secondary to the primary cause of failure, namely, crevice 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. 

Oxidation of a metal or alloys to its (lower energy state) oxides or cations. In effect, the wastage or other damage to a metal caused by one or more of several types of chemical or electrochemical reactions. Takes many forms such as galvanic, crevice, pitting, underdeposit, and biologically induced corrosion. 

Anodic undermining has not been studied as extensively as cathodic delamination because there do not appear to be any mysteries. Galvanic effects and principles which apply to crevice corrosion provide a suitable explanation for observed cases of anodic undermining. 

Figure 34 The steps involved in determining the depth of container wall penetration under Canadian nuclear waste disposal conditions using data obtained in an electrochemical galvanic coupling experiment. (A) Crevice propagation rate (R cc Ic) as a function of temperature (T) (B) RCc as a function of 02 concentration [02] (C) calculated evolution of container surface temperatures and vault 02 concentrations with time in the vault (D) flux of 02 (Jo2) to the container surface as a function of time (E) predicted evolution of Rcc up to the time of repassivation (i.e., at [02]p) (F) total extent of crevice corrosion damage expressed as the total amount of 02 consumed (Q) up to the time of repassivation (G) experimentally determined maximum depth of wall penetration (Pw) as a function of 02 consumed (Q) (H) predicted maximum value of Pw up to the time of repassivation (fP)-...

What is clear from these analyses is that the avoidance of crevice corrosion will delay eventual container failure significantly, irrespective of whether it occurs by wall penetration or by HIC. With this is mind, the galvanic coupling technique (along with the associated analytical methods outlined above) can be used to compare qualitatively the crevice corrosion performance of a series of titanium alloys. Figs. 36A and B compare the parameter (/c, Ec, Ep) values ob-... 

After selection of the material, the corrosion scientist must play a role in designing the equipment so that the design is appropriate, and avoids corrosion modes due to inappropriate design. Improper design may result in galvanic corrosion, crevice corrosion,... 

The various modes of degradation encountered are (i) general or uniform corrosion (ii) galvanic corrosion (iii) pitting corrosion (iv) crevice corrosion (v) filiform corrosion (vi) granular corrosion (viii) stress corrosion cracking (viii) corrosion fatigue. 

Macroscopic forms of corrosion affect greater areas of corroded metal and are generally observable with the naked eye or can be viewed with the aid of a low-power magnifying device. Macroscopic examination can identify the following forms galvanic, erosion-corrosion, crevice or pitting, exfoliation, and dealloying. Microscopic... 

The increase in cathodic kinetics due to the action of biofilms on passive alloy surfaces can also increase the propagation rate of galvanic corrosion. Potentiodynamic polarization studies show that cathodic kinetics are increased during biofilm formation on passive alloy surfaces. Tests on crevice corrosion samples of passive alloys S30400 and S31600 revealed that crevice initiation times were reduced when natural marine biofilms were allowed to form on the exposed external cathode surface. (Dexter)5... 

In the chambers crevice corrosion and galvanic corrosion along with general corrosion of the aluminum ladder were observed (Figure 7.2). Crevice corrosion was also observed on couplings and bolts with valves. The aluminum steps in some valve chambers were attacked by chlorides/phosphates. 

Rare earth metal salts Aircraft industry. Effective for pitting corrosion, stress corrosion cracking, corrosion fatigue, galvanic corrosion and crevice corrosion by using rare earth metal salts in waters for washing aircraft. Disposal of the waters is environmentally safe [4,6]... 

For crevices such as in those in socket welds, the metal in the crevice is likely to be anodic. Crevice corrosion and under-deposit corrosion can be serious problems in oxide-stabilized materials such as aluminum and the stainless steels. Crevices and deposits can also accelerate corrosion in metals (such as carbon steel) that do not exhibit both active and passive states. However, the rate of corrosion is much slower in such materials because they lack the galvanic driving force of the active-passive states characteristic of the oxide-stabilized metals and alloys. The anode areas in crevices and under deposits are typically smaller than the cathode areas. This difference accelerates the corrosion rate. 

Electrochemical potentials can arise from differences in electrolyte or electrode concentration as well as from differences in chemical composition. Thus, for example, there will be a difference in potential between two amalgam or alloy electrodes of the same basic type in which there is a difference in the activity of one of the alloy or amalgam constituents. The practical implication is that galvanic corrosion can occur between similar alloys of different composition. Crevice corrosion phenomena are often explainable in terms of differences in oxygen concentration. 

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