Testing a material's corrosion response to a seawater environment at temperatures up to 200°F (93.3°C) with hypochlorite additions up to 10 ppm (part per million by volume) must be done carefully or the results will not be accurate. Hypochlorite thermally degrades rapidly, essentially removing it from the testing solution during the test, invalidating the results. Using potentiodynamic scans and corrosion potential measurements, it is possible to overcome the problem of hypochlorite degradation and determine the pitting and crevice corrosion susceptibility of a material. This paper examines the method and provides examples of its use with several passive metals (UNS N10276, S31603, S31254, and R50400).
Hypochlorite is a common biocide used in seawater injection systems at concentrations up to 10 ppm (part per million by volume), often through continuous injection from a seawater hypochlorite generator. Because it is a powerful oxidizer, testing is often required to determine changes in a material's susceptibility to pitting or crevice corrosion due to the presence of these low concentrations of hypochlorite, especially at elevated temperatures, such as that encountered in seawater cooled heat exchangers. The initial attempt to test in substitute seawater with hypochlorite at temperatures up to 200°F (93.3°C) revealed a very rapid decomposition of the hypochlorite ion during the test. A valid test requires that the environment remains constant long enough to measure the effect. Thus, a test method was developed that accounted for the transient nature of the hypochlorite.
Hypochlorite ions (OCl-) decompose through two pathways, one that forms chlorate (ClO3) and another that forms O2.1,2 The formation of oxygen is a very slow reaction, given in equation 11, and is often catalyzed by the presence of metal ions.
(mathematical equation available in full paper)
The decomposition to chlorate follows the following mechanism1:
(mathematical equation available in full paper)
The rate determining step of the decomposition from hypochlorite to chlorate (equation 2) has been shown to be both a second and third order, depending upon the pH. For a pH range of 5-9, where seawater is approximately 8 at ambient temperature, the reaction is third order3. The rate of the reaction is also determined by the temperature and the initial concentration of the hypochlorite ion. The complexity of accurately determining the rate constant (k) makes calculating the concentration of hypochlorite remaining in the solution at a given time difficult, at best.
This method is based on a combination of cyclic potentiodynamic polarization and the measured free corroding potential of a particular passive material in a specific environment. The cyclic potentiodynamic test involves polarizing a material in the test solution cathodically to a set potential, scanning the potential anodically at a constant rate to a set potential and reversing the scan to return to the original cathodic potential. The resulting plot of current as a function of the applied potential reveals a number of characteristic features that are related to the active/passive behavior of the alloy in the test environment. An idealized plot is shown in Figure 1. The scans are conducted broadly according to ASTM G 61.4
