Galvanic corrosion assembly

Mechanical components can take various forms. Insulation spacers can be used between components made from different metals. The thickness of these spacers can be varied to alter the insulation distance between the anode and cathode. Additionally, the solution him depth can be different. Furthermore, there can be more than one galvanic couple such as many steel fasteners for one magnesium part, and there can be an interaction of the current caused by each galvanic couple. Waber [8] studied the influence of galvanic cell size theoretically, but had no experimental validation. Song et al. [4] developed a galvanic corrosion assembly (GCA) to study the influence... 

Section through multi-electrode Mg-steel galvanic corrosion assembly (GCA) [17]. 

Fig. 9,20 Spiral bevel ring gear assembly from the forward transmission gear box of Boeing Vertol 234-LR (Chinook) aircraft registration G-BWFC which crashed into the sea off the Shetland Isles in 1986. Note the peripheral and radial fractures in the gear, which appeared to be responsible for the crash. There was evidence of fretting and galvanic corrosion which may have been responsible for initiation of the fracture sequence...

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. 

The test assembly used originally by Subcommittee Vlll of ASTM Committee B-3 in its comprehensive studies of atmospheric galvanic corrosion had the disadvantage that it depended on paint coatings to confine corrosion to the surfaces in actual contact with each other. In interpreting the results, it was frequently difficult to decide how much corrosion was due to galvanic action and how much to a variable amount of normal corrosion through failure of the paint system. 

Galvanic corrosion testing under atmospheric exposures has been standardized and bears the titles ISO 7441-84, ASTMG 104-89 for plate tests and ASTM G116-93 for wire-on-bolt tests.21,34,35 A form of test assembly for galvanic corrosion testing is shown in Figure 2.5. 

Galvanic corrosion may occur in carbon fiber composites if aluminum fasteners are used, due to the chemical reaction of the aluminum with the carbon. Coating the fastener guards may prevent corrosion but adds cost and time to the assembly process. As a result, aluminum fasteners are often replaced by more expensive titanium and stainless steel fasteners in carbon fiber composite joints. 

The causes of corrosion are complex. As in the case of chemical attack, adhesive bonding has been shown to offer better protection against service corrosion and galvanic corrosion than many other mechanical assembling techniques (Figs 16 and 17). 

FIG. 13—Plate test assembly for galvanic corrosion testing (redrawn and simplified from ISO 7441). 

FIG. 16—Cross section of disk test assembly for atmospheric galvanic corrosion. 

FIG. 3—WIre-on-boK assembly for measuring atmospheric corrosivity and/or galvanic corrosion. 

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]. 

Electrochemically Induced Failures. The electrochemical failure mechanisms accelerated by temperature, humidity, and electrical bias that were described in Sec. 57.2.1.3 for printed circuit boards also apply to the remainder of the PCA. The solder used for the interconnects and the metal component terminations and lead frame finishes can also be involved in the reactions. The large number of dissimilar metals increases the complexity of the situation and the possibility of galvanic corrosion in a humid environment. In addition, contaminants introduced during printed circuit assembly such as flux residues can contribute to the failures. 

For common metals, dissolution potential scales similar to those in Table B.1.3 are available. It is always necessary to specify the reference electrode and the medium in which the measurement was taken. Dissolution potentials allow classifying metals with respect to each other, which is useful for the prediction of galvanic corrosion in heterogeneous assemblies (see Chapter B.3). 

Crevice corrosion associated with galvanic corrosion may develop under steel washers (ordinary or stainless) used for mounting aluminium wall panels exposed to the seashore [41]. For this reason, assemblies must be mounted with great care in such an aggressive environment in order to avoid the formation of zones that retain humidity, which would favour galvanic and crevice corrosion. 

For a very long time, galvanic corrosion of aluminium in contact with other metals was a major concern to users, to such an extent that it slowed down the development of applications of aluminium whenever the issue of its behaviour in contact with other metals was raised. Nowadays, it is surprising to discover the extensive precautions that were taken in former times with heterogeneous assemblies exposed to ambient atmosphere. 

According to the ranking of aluminium and its alloys on the potential scale (Table B. 1.4), in virtually any assembly with other common metals, aluminium will be the anode of the resulting cell and hence likely to suffer galvanic corrosion, if the conditions are favourable. 

On bolted aluminium-steel assemblies with unprotected contacts, an important accumulation of alumina is observed in the contact area after a few years of immersion in seawater. This accumulation is capable of deforming the aluminium sheets. This cataplasm may stop galvanic corrosion, at the price of a certain deformation of the aluminium sheets. 

It is obviously not appropriate to rely on possible anodic or cathodic polarisation phenomena in order to limit the damage caused by galvanic corrosion. Prevention is necessary at the assembly design stage. 

The most favourable case is a very large anodic surface and a small cathodic surface, for example, an aluminium sheet of large surface area mounted with a few bolts made of stainless steel. Experience shows that if this assembly is immersed, severe galvanic corrosion will occur in the contact area with the stainless steel bolts. 

Zones that permanently retain humidity such as low points, basins, i.e. the hidden recesses where wet dust or mud accumulates, are sites where galvanic corrosion may develop. This happens especially in hidden recesses of vehicles at bolted or riveted assemblies between steel and aluminium where mud projected from the road may accumulate. 

As mentioned above, in the case of an assembly with steel, the accumulation of corrosion products (alumina) may slow down or even stop galvanic corrosion. However, sometimes this happens only after serious damage to the aluminium parts has already occurred. 

Landing stages of marinas are assembled with bolts made in ordinary or stainless steel. The inspection of this equipment shows that no galvanic corrosion can be detected with the... 

On the scale of potentials, zinc is more electronegative than aluminium, while cadmium has a potential very close to that of aluminium. Galvanised or cadmium-coated steel fasteners can, therefore, be used to join and assemble stmctures made from aluminium alloys. It should just be remembered that when these coatings become too worn to protect the steel and the aluminium, the previous scenario applies in which there is contact between the aluminium alloy and bare steel. Chromium-plated steel does not lead to galvanic corrosion with aluminium, as long as chromium covers the nickel underlayer however, the contact between aluminium and the nickel underlayer would lead to galvanic corrosion of aluminium. 

Of course, this is not an issue for immersed assemblies that must be protected in any case. Several methods are available for the measurement of galvanic corrosion between aluminium and other metals or alloys. 

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