High Strength HISC-Resistant Bolt Materials for Seawater/Cathodic Protection Service
Cathodic protection is often used to inhibit the corrosion of metals used in seawater environments. Unfortunately, this process produces hydrogen which diffuses into the metal and causes hydrogen embrittlement.
CAMP Professors Daryush Aidun, Ian Suni, and David Morrison along with graduate students Marissa LaCoursiere and Josiah Jebaraj have initiated a research project to develop high strength bolt materials that resist hydrogen-induced stress cracking (HISC) when subjected to a cathodic potential in a seawater environment. The project is sponsored by General Electric Oil & Gas / Vetco Gray.
Graduate student Josiah Jebaraj performs hydrogen permeation experiments.
The group has completed the first round of testing which consisted of experiments to validate their experimental systems. The tests were accomplished on 4340 steel, an alloy that has been shown to be susceptible to hydrogen embrittlement when heat treated to a high strength condition. The group performed hydrogen permeation experiments using the Devanathan-Stachursky method and determined the diffusion coefficient of hydrogen in 4340 steel using multiple methods. Mechanical property characterizations were accomplished by performing slow strain rate tests at a strain rate of 1x10-6 s-1. Control environment tests were performed in deionized (DI) water at 4°C without a cathodic potential, and service environment tests were performed in simulated ocean water at 4°C with a cathodic potential of -1100 mv. Figure 2 shows that annealed 4340 steel exhibited a small loss of ductility in the simulated ocean environment, but the hardened alloy was severely embrittled when subjected to the simulated ocean environment.
All of the experiments provided a positive validation of the experimental systems and methods. The group will now investigate alloys that possess chemical and microstructural characteristics that are expected to lead to enhanced resistance to hydrogen embrittlement.
Figure 2. Slow strain rate tests on annealed and hardened 4340 steel.