Atomic hydrogen (H+), generated by the cathodic corrosion reaction can lead to two types of failures in steels; Hydrogen Blistering and Hydrogen Induced Cracking (HIC). Part of the hydrogen produced by the cathodic corrosion reaction may permeate the steel by diffusion, and the balance is dissolved in the process electrolyte or released as bubbles of gas. Poisoning agents in the process, electrolyte or released as bubbles of gas. Poisoning agents in the process, such as hydrogen sulfide, cyanide, and arsenic, inhibit the formation of molecular hydrogen on the metal surface and therefore increase the proportion of atomic hydrogen which diffuses into steel. Hydrogen monitoring instruments indicate the susceptibility of steel to HIC and hydrogen blistering by measuring the rate of atomic hydrogen permeation. Generally, the utility of this application is in qualitatively detecting process upsets which tend to markedly increase the flux of nascent hydrogen. Because the hardness and stressing of the steel affects the susceptibility to hydrogen damage, quantitative assessment is often difficult. However, relative hydrogen flow rates and the estimated associated damage they create are available (See RCS Application Note AN111 for further guidance). Hydrogen monitoring is also used on occasions to monitor the rate of corrosion, but the relationship between corrosion rate and hydrogen permeation varies with fluid dynamics, the method of hydrogen monitoring, and relative content of poisoning agents in the process.
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