Theoretical and Experimental Aspects of the Corrosivity of Simulated Soil Solutions

Corrosion of buried steel pipe is a permanent engineering problem and, albeit the counter measures against degradation, when the corrosion process takes place, the damage has costly impact. In order to study the corrosion behavior of pipelines, it is possible to use actual soil extracts or simulated soil solutions. The extract is much related to specific sites and consequently too strict to permit a general understanding. The simulated soil presents, as advantage, its inorganic characteristic and easy preparation. In this paper, we present some theoretical results concerning the chemical equilibria of NS1, NS2, NS3, and NS4 simulated soil solutions. Besides, we have studied the effect of the above four media in corrosion behavior and polarization curves were performed for an API 5L X65 steel. The theoretical findings show that each ionic concentration varies for a 6–12 pH range. The experimental data suggested that the corrosion currents decrease as high is the pH and increase as high is the chloride content. Notwithstanding these facts, for multielectrolyte solutions, a simple correlation with a given ion is not straightforward but the complementary approaches used here give useful insights. 1. Introduction Carbon steels of buried pipelines are susceptible to degradation by soil corrosivity. Therefore, this situation causes worldwide safety and economy concerns. The uses of coating and cathodic protection are the standard procedures. However, during in-service period, this coating can suffer localized failures and the steel is exposed to a corrosive environment. Generally, the degradation occurs as transgranular or intergranular stress corrosion cracking or even as hydrogen embrittlement in carbon dioxide environment because of mechanical stress. Few works are devoted to investigate corrosion processes caused by aqueous solutions without carbon dioxide. In this sense, marked reductions in area were observed near corrosion potentials of API 5L X65 when performed in slow strain rate test [1]. This fact shows that, even without CO2, the corrosion behavior close to free potential is related to stress corrosion cracking. To study the corrosivity of soil environments by ordinary electrochemical methods, two media are frequently used—extract solution from soil samples and simulated ones. The first option is very complex because it can contain, besides inorganic species, organic chemicals and even bacteria. Albeit their high correlation with the site where the pipe is buried, its use is not applied everywhere precisely because of inherent local