Galvanic corrosion factors

Critical factors. Two factors are critical in galvanic corrosion of weld metal. The first is the existence of sufficient compositional differences... 

Elimination. Recall that the critical factors governing galvanic corrosion of welds are the presence of substantial compositional differences within the weld metal and the exposure of such a weld to a sufficiently aggressive environment. If the aggressiveness of the environment cannot be sufficiently reduced, significant compositional differences within the weld metal must be avoided. This requires following proper... 

Galvanic corrosion is location specific in the sense that it occurs at a bimetallic couple (Fig. 16.2). It is metal specific in the sense that, typically, corrosion affects the metal that has less resistance in the environment to which the couple is exposed. Hence, in principle, we would anticipate galvanic corrosion of relatively reactive metals wherever they are in physical contact with relatively noble metals in a sufficiently aggressive, common environment. Experience has shown, however, that all such couples do not necessarily result in unsatisfactory service. This is because of the interplay of various critical factors that influence galvanic corrosion. These critical factors are discussed in the next section. 

Galvanic corrosion in typical industrial cooling water systems is the net result of the interplay of these factors. Some factors may accelerate the corrosion process others may retard it. In their approximate order of importance, the more influential factors are discussed below. 

Figure 4.17 illustrates the corrosion occurring on high-purity AZ31 and ZW3 in contact with steel bolts. Tested alone in sea-water, the corrosion rate of the former is much the lower. It is evident from the illustration, however, that the governing factor in galvanic corrosion is the type of electrolyte present rather than the composition of the alloy. 

Corrosion and poor installation are by far the most common causes of storage system leaks. The most common causes of release from bare-steel UST systems are galvanic corrosion and the breakdown of hard refined steel to its natural soft ore. Because older USTs are usually constructed from bare steel, corrosion is believed to be the leading factor contributing to release. The speed and severity of corrosion varies depending on site characteristics, such as soil conductivity,... 

There are many permutations of galvanic corrosion that can occur when two or more dissimilar metals or alloys are coupled in a cooling water electrolyte. The less noble metal actively corrodes, the extent of which is determined by a number of factors. 

The conductivity and the composition of the medium are controlling factors in the extent of galvanic corrosion, as evidenced by the corrosion rates in 3% NaCI and tapwater (see Table 4.71). Cadmium-plated steel gives a lower corrosion rate than steel. 

Factors Involved in Galvanic Corrosion. Emf series and practical nobility of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of solution potentials since it allows to classification of the metals in order of nobility according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/1). This thermodynamic nobility can differ from practical nobility due to the formation of a passive layer and electrochemical kinetics. 

Fig. 1.6 Factors controlling galvanic corrosion [21], This material is reproduced with permission of John...

Some of the other factors affecting galvanic corrosion are area ratios, distance between electrically connected materials, and geometric shapes. Galvanic corrosion of the anodic metal takes the form of general or localized corrosion, depending on the configuration of the couple, the nature of the protective films formed and the nature of the metals. 

With these factors in mind, disc type coupons were chosen. These were similar to the American Sodety for Testing and Materials (ASTM) test coupons but smaller in diameter (100 and 70 mm) to facilitate transport. Single sheet coupons were used to evaluate pitting corrosion on exposed surfaces, and couples or sandwich type coupons were used to simulate crevice and/or galvanic corrosion. The coupons were cleaned and numbered, and the alloy type was identified using a laser scriber. The coupons were stacked in a stainless steel rack. This consisted of a steel pipe welded to a stainless steel base plate and threaded at the other end to hold the coupons in place with a stainless steel nut welded to a hook to enable the rack to be suspended with a nylon rope. A... 

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