L51426 Studies to Evaluate Inhibitor-Containing Primers for Controlling Stress-Corrosion Cracking

Abstract

Studies to develop methods for preventing stress-corrosion cracking have been concentrated in three general areas: (1) modifying the steel from which pipes are made, (2) modifying the operating practices involving stress, temperature and cathodic protection, and (3) modifying the environment. In the latter category, the main approach has been the use of inhibitors to modify the environment known to promote stress-corrosion cracking. In early experiments in the laboratory, a number of chemicals were identified as being good inhibitors of stress-corrosion cracking for line-pipe steel under conditions that were thought to exist at a pipeline. Very good protection was obtained, for example, in solutions containing certain chromates, phosphates or silicates as inhibitors. However, in studies to simulate soil conditions, addition of soluble phosphate to the soil above a buried pipeline was shown to be ineffective because of several factors, including the slow migration of the phosphate ions through the soil to the pipe surfaces and precipitation as calcium phosphate through reaction with the calcium ions present in the soil. This report summarizes the results of field studies, supporting laboratory experiments and the modeling of inhibitor loss through leaching. The field exposure of coated pipe samples lasted '2 years and provides significant data for the evaluation of the inhibited primer-plus-coating concept for controlling stress-corrosion cracking under simulated practical conditions. As a tool for evaluating the field data, in a parallel effort, mathematical models were developed for the calculation of the movement of ions influenced by diffusion and cathodic protection at and near a pipe surface. These models also were used to estimate the useful life of inhibitors in coatings under several practical scenarios.