CuNi Alloys in Chlorinated Substitute Ocean Water. Effect of Sulfide in Surface Conditioning with Substitute Ocean Water

Six weeks exposure tests were run with CuNi 70/30 (C71500) and CuNi 90/10 (C70600) tubes in flow loos with substitute ocean water (ASTM D1141; pH 8.2; flow rate 0.05 m/s) at room temperature and 40°C in the absence and presence of 1 ppm hydrogen sulfide. The effect of sensor controlled hypochlorite concentration (0.5; 1.0; 3.0 ppm) was investigated on tube surfaces i) in the as-received surface condition (surface state A), ii) after conditioning for 6 weeks in additive-free substitute ocean water  (surface state B) and iii) after conditioning for 6 weeks in substitute ocean water containing 1 ppm hydrogen sulfide (surface state C). The conditioning was performed at room temperature and access of air to the conditioning medium.

In the surface states A and B the corrosion intensity increases at both types of CuNi alloys with increasing hypochlorite concentration (1 and 3 ppm). This stimulating hypochlorite effect is more pronounced at CuNi 70/30 than at CuNi 90/10, but hardly visible in the presence of only 0.5 ppm hypochlorite. In the presence of hypochlorite CuNi 90/10 was found to be susceptible to denickelification which was favoured with increasing hypochlorite concentration. This effect appeared to be more intense at surface state A than at surface state B.                                 

Independent of the surface states A, B or C and/or the test temperature (RT, 40°C) 1 ppm hydrogen sulfide in the substitute ocean water increases the corrosion intensity at CuNi 90/10 significantly. The protectivity of scales present in surface state B is markedly decreased. The highest corrosion intensity was found when CuNi 90/10 surfaces in state C were exposed to hydrogen sulfide containing substitute ocean water. These findings are  basically valid also for CuNi70/30. However, the corrosion rates are significantly higher than at CuNi 90/10.

It appeared that  in case of CuNi 70/30 the poor protectivity of surface state C is improved by adding 1 or 3 ppm hypochlorite to the flowing substitute ocean water at RT and 40°C. This effect was not seen in case of CuNi 90/10.  In the surface state A or B,  CuNi 70/30 corrodes less at 40°C than at RT when exposed to hydrogen sulfide containing substutite ocean water. However, in surface state B this temperature effect is not so pronounced.