Galvanic corrosion inhibition

The most serious form of galvanic corrosion occurs in cooling systems that contain both copper and steel alloys. It results when dissolved copper plates onto a steel surface and induces rapid galvanic attack of the steel. The amount of dissolved copper required to produce this effect is small and the increased corrosion is difficult to inhibit once it occurs. A copper corrosion inhibitor is needed to prevent copper dissolution. 

Corrosion prevention—material selection, corrosion inhibition, cathodic protection, galvanic corrosion prevention, and extensive nondestructive and destructive inspection protocols. ... 

Lead based solders (tin/lead solders) contain up to 60 % of lead, which can be released into water through galvanic corrosion. The corrosion rate is increased by high concentrations of chloride and nitrate but is inhibited by sulfate, silicate and orthophosphate. Lead concentrations at the tap depend not only on the corrosion rate but also on the number of leaded joints in the plumbing, the area of solder exposed to water at each joint, and the water usage pattern (Gregory, 1990). 

Use Level 2-30% (corrosion inhibition), 10-20% (zinc-contg. paints), 4-12% (coatings for use over galvanized steel), 2-9% (flame retardance), 4% (uv stabilization)... 

Uses Adhesion promoter for baked alkyds, polyesters, acrylics, and lacquers onto metal substrates corrosion-inhibiting pretreatment or base coating for galvanized steel and aluminum suitable for food-contact coatings... 

The ability of CeCls to inhibit galvanic corrosion between other dissimilar metal couples has also been investigated (Hinton 1989). The efficiency of inhibition (fig. 9)... 

Hinton et al. (1988) have immersed a copper-mild steel galvanic couple in tap water containing 200 ppm CeCb for 20 hours until the Ce oxide film was observed on the copper cathode. The couple was then removed, washed and immersed into tap water without any CeCb. In this instance the galvanic current remained virtually unchanged from the equilibrium value which was observed before the switch in test solutions. This indicates that the R oxide films, which form on cathodic areas, continue to inhibit galvanic corrosion even after the removal of the inhibiting cations from solution. 

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. 

Similar general and galvanic corrosion tests were carried out for some other coolants with KF as an inhibitor. The results are summarized in Table 11.5. A comparison of Table 11.5 with Tables 11.2 and 11.3 leads to a conclusion that KF has an inhibition effect in all of these coolants. LLC-F -I- 1 wt% KF has acceptable corrosivity to AM-SCl alloy and LLC-T -I- 1 wt% KF is also close to the acceptable level. 

The question one has to put is that, if this is the case, would copper then continue to deposit and nickel completely dissolve from the deposit However, snbseqnent analysis showed that copper emichment at the surface slowed down nntil the copper content reached about 80%, after which the reaction virtually stopped (Roy and Landolt, 1995 Bradley e/a/., 1996). The galvanic corrosion is controlled by copper mass transfer through the hquid phase at the beginning, showing a linear growth of the Cu-rich layer (Roy and Landolt, 1995). Since the reaction stops due to surface emichment of copper, the growth is inhibited quite suddenly when the thickness of the layer reaches about lOiun (Roy and Landolt, 1995 Bradley et al, 1996). 

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