Galvanic corrosion, described

An obvious method for studying galvanic corrosion either with or without supplementary electrical measurements is to compare the extent of corrosion of coupled and uncoupled specimens exposed under identical conditions. Such measurements may use the same techniques for estimating corrosion damage, such as mass-loss determinations, as have been described in connection with ordinary corrosion tests. 

The most common overcoats, however, are sputtered carbons. Their role in disk corrosion has been described in contradicting ways. Whereas Garrison [141] clearly observed that carbon, like Rh, can enhance galvanic corrosion, Smallen et al. [131] believe that carbon decreases corrosion by preventing lateral growth of corrosion products. Results of similar tests are sometimes contradictory Nagao et al. [145] have shown an improvement of the corrosion resistance of carbon-coated CoCr alloys on T/H test (with either SOz gas or NaCl mist), whereas Black [146] finds that pyrolitic carbon over a CoCrMo alloy results in elevated corrosion rates. 

Nevertheless, there were reliable reports of corrosion occurring with molybdenum disulphide in films and in greases. Several such reports arose from the US Army , originating with a salt fog test of a missile launcher in which all parts coated with solid film lubricant rusted badly. Subsequent reports described galvanic corrosion of various metals with molybdenum disulphide in moist atmospheres. 

The anode and cathode corrosion currents, fcorr.A and fcorr,B. respectively, are estimated at the intersection of the cathode and anode polarization of uncoupled metals A and B. Conventional electrochemical cells as well as the polarization systems described in Chapter 5 are used to measure electrochemical kinetic parameters in galvanic couples. Galvanic corrosion rates are determined from galvanic currents at the anode. The rates are controlled by electrochemical kinetic parameters like hydrogen evolution exchange current density on the noble and active metal, exchange current density of the corroding metal, Tafel slopes, relative electroactive area, electrolyte composition, and temperature. 

The differential chemical reactivity of the Cu and barrier films when exposed to the slurry chemicals in the polishing environment can lead to the desired selective material removal but can also generate a variety of defects—corrosion pits, fangs due to galvanic corrosion, etc., and the underlying processes can be best investigated using a variety of electrochemical techniques, as described in the chapter authored by Dipankar Roy. 

Galvanic corrosion is defined and described in Section IV of this manual on corrosion types. 

Galvanic corrosion tendencies can be evaluated by developing a galvanic series for the materials of interest as described in ASTM G 82. Galvanic corrosion rates can be established by exposure of galvanic couples as described in ASTM G 71. 

Three standardized atmospheric galvanic corrosion tests are the plate test, the wire-on-bolt test, and the disk test. The plate test, described in ISO Standard IS07441, Determination... 

The wire-on-bolt test, described in ASTM G 116 (Practice for Conducting the Wire-on-Bolt Test for Atmospheric Galvanic Corrosion), has been used with standard materials as an atmospheric corrosivity test under the names of the CLIMAT and ATCORR tests. This test consists of wrapping a 1.0-m length of wire of the anodic material around a threaded bolt or rod of the cathodic material. Post-test evaluation is typically done by mass loss only. Since the wire diameter is much smaller than the 0.5-cm galvanic interaction distance in the atmosphere, the effective tmode-to-cathode area ratio is well below 1 1, making this a fast test. A typical exposure duration is 90 days. The short duration is... 

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. 

In a very concise review, Welty described the entire research and development process and results that lead to the EPRI GOG procedures. Since these have been discussed at length above, the discussion will not be repeated. Two areas of concern, the effect of residual sulfur from CCI-801 and galvanic corrosion to various weldments, have been the subject of continued work. 

The inhibition mechanism of fluorides described above also operates on magnesium alloys in commercial coolants. For example, the inhibition effect of KF on the general corrosion of AM-SCl in MBL coolant at room temperature is illustrated in Fig. 11.10. The general corrosion rate of AM-SCl decreases as the concentration of KF increases. When the concentration of KF is greater than 1 wt%, the corrosion rate becomes lower than the corrosion rate threshold of 0.67 mg/cm /week [26]. However, for galvanic corrosion at high temperature, KF does not work very well in the MBL coolant. 

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