Crevice corrosion galvanic effects

U-bend and double U-bend specimens LP turbines, piping, feedwater heaters, condensers, boilers Detects general corrosion, pitting, and SCC double U-bends simulate crevice and galvanic effects... 

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. 

An idea of the.diktributibh bf galvanic corrosion in the atmosphere is prp vided by the location of the corrosion of magnesium exposed in intimate contact with steel in the assembly shown in Fig. 19.28 after exposure in the salt atmosphere 25 m from the ocean at Kure Beach, North Carolina, for 9 years. Except where ledges or crevices may serve to trap unusual amounts of electrolyte, it may be assumed that, even with the most incompatible metals, simple galvanic effects will not extend more than about 4-5 mm from the line of contact of the metals in the couple. 

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. 

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]... 

Two subtle corrosion effects can occur when a single metal is in contact with an electrolyte -differential aeration and crevice corrosion. Differential aeration can cause corrosion when no obvious galvanic cells are in evidence. To illustrate this effect, suppose we have a cell with a copper anode and cathode. If the concentration of the electrolyte and the temperature of each cell compartment is the same, no potential is generated and no corrosion occurs. However, bubble O2 into the one compartment, which becomes the cathode compartment, and corrosion will occur in the other, which forms the anode compartment. Differential aeration is, in fact, a concentration effect, and can be understood by using the Nernst equation. Electrons will flow from anode to cathode and the anode will corrode. 

Duplex stainless steel Utilised in the oil, gas and petrochemical industry for critical equipment construction where erosion and corrosion resistance is required. However, it can be susceptible to galvanic and crevice corrosion effects. Manufacturing difficulties and repair capability, combined with high cost, limit the use of this material for large constructions. 

The remote crevice assembly technique (see Chapter 19) is a research tool that allows one to separate the anode and cathode areas of a crevice corrosion test sample so that the current flowing between them can be measured with a zero-resistance ammeter. This technique is similar to the dual cell method, and it lends itself well to studies of microbial effects on crevice corrosion [7]. It allows direct measurement of microbial effects on both the initiation time and propagation rate for crevice attack, provided again that a suitable control experiment without the microbial influence can be done concurrently. The scime technique of separating the anode and cathode can be used to study the influence of microbes in biofilms on galvanic corrosion [li]. 

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. 

The importance of concrete cracks in rebar corrosion has also been highlighted by Niirnberger. Both carbonation and chloride ion diffusion, two important processes associated with rebar corrosion, can proceed more rapidly into the concrete along the crack faces, compared with uncracked concrete. Niirnberger argued that corrosion in the vicinity of the crack tip could be accelerated further by crevice corrosion effects and galvanic cell formation. The steel in the crack will tend to be anodic relative to the cathodic (passive) zones in uncracked... 

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