Galvanic couples, exposure

The specimen design used in the study by Rostoker et al. was such that it simulated both galvanic coupling and crevice conditions. Specimens were immersed in a 1% saline solution at 37 C, and examined by optical microscopy after exposures of a few to 100 days. No corrosion was observed on Ti-6A1-4V when the alloy was either uncoupled, coupled with itself (simple crevice). Or coupled with type 316L stainless steel, cast Co-Cr-Mo... 

Figure 49 Electrochemical time series of galvanically coupled AA2024-T3 (ST) in 1 M NaCl. No deaeration and 0.01 cm2 exposure area, (a) Current and (b) open circuit potential. The horizontal dashed lines represent the mean pitting (top) and transition (bottom) potentials of high purity A1 at this Cl- concentration. (Data courtesy of Sheldon T. Prude, University of Virginia.)...

Figure 10. Visual comparison of stainless steel/painted auto-body steel galvanic couples, one season exposure. Right to left Boston, Dallas, Detroit, Montreal.

Rack 1 was inserted on 12 November 1996 and was withdrawn for a short time on a monthly basis for visual inspection. No pitting corrosion was seen on any coupons during the 12 inspections. The conductivity of the water ranged from 0.1 to 0.8 pS/cm, with a pH between 4.8 and 6.1. The chloride ion content was <0.5 ppm. Racks 2 and 3, which contained two galvanic couples and three crevice sandwiches, were exposed in October 1998. Rack 2 was withdrawn after one year of exposure, in October 1999. The conductivity of the basin water was always <1 pS/cm, the chloride ion content was <0.05 ppm and the pH was 5.5-6.S. 

Three additional racks were removed from the L basin and from the RBOF in early 1998 after two years of exposure to water with a conductivity of 1-3 pS/cm and a chloride ion content in the ppb range. No pitting was observed on the other 180 coupons in these racks. Some mild oxidation of the surfaces was seen on all coupons. Some mild surface reaction was seen in the crevice area of the stainless steel-aluminium galvanic couple coupons. A number of the large aluminium coupons had areas on which the surface appeared to be protected from oxidation during immersion in the basin water. These areas can be seen in Fig. 1.7. In these areas, the aluminium surface was similar in colour to the areas of the specimen covered by an insulator or between crevice specimens that had not been exposed to the water. The remainder of the coupon surface exposed to the water was dark grey in colour. 

Though the original information package for the proposed CRP specified six base metals, two crevice sandwiches and four galvanic couples for exposure, the racks of Batch I contained fewer coupons. 

Gallaccio, A., and Comet, I. (1960). Report on Marine Atmosphere Exposure of Galvanic Couples Involving Magnesium, ASTM STP 255. ASTM, Philadelphia, 26... 

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. 

Duration of exposure is extremely important in galvanic corrosion testing. In many galvanic couples, the amount of corrosion is highly variable with time. Initial high corrosion rates may fall dramatically, initial low rates may suddenly increase, and the relationship between anodic and cathodic materials can even reverse with time. For these reasons, the duration of the test exposure should be of sufficient length to ensure that the corrosion processes have reached steady... 

Immediately before and after the exposure, the contact resistance between the metals in each galvanic couple is checked with an ofammeter. Following exposure, initial cleaning of marine growth is done with a plastic scraper and nylon brush, then transferred to the laboratory for final cleaning. 

After some days of exposure (Fig. 7.11(b)) the whole grains are corroded away, mainly due to the galvanic coupling between the a-phase and the eutectic a-/p phase, which results in the formation of deep pits which can be seen after a few days of exposure (15). The corrosion product magnesite is hydrated into nesquehonite. It is also possible that brucite starts to form due to a low CO2 content at the bottom of the pit at this stage. 

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