Once a good surface film forms, the corrosion rate will continue to decrease over a period of years and to exhibit the classical parabolic growth rate of protective layers. For this reason, it has always been difficult to predict the life of copper-nickel based alloys based on short-term exposures. Normally, corrosion rates of 0.02-0.002 mm/yr are anticipated.

Specific corrosion rate measurements at LaQue Corrosion Services taken over fourteen years in tidal, flowing (0.6m/s) and quiet seawater show the corrosion rate decreases over a period of 5-6 years, stabilising out at about 1.3µm/yr, as shown in Figure 2.

FIGURE 2. The change in corrosion rate with time for 90-10 and 70-30 copper-nickel in quiet, flowing and tidal zone seawater.

The presence of sulfides in polluted water and the decomposition of organic matter may lead to higher corrosion rates and pitting, as will be discussed later.