High Temperature Hydrogen Attack
High-Temperature Hydrogen Attack (HTHA) is a damage mechanism that affects equipment that has been exposed to hydrogen at certain temperatures and pressures. At these temperatures, the hydrogen molecules (H2) dissociate into individual hydrogen atoms. These atoms are small enough that they are able to dissolve into the steel.
Once the hydrogen gets in, it reacts with carbon in the steel to form methane. Unlike the hydrogen, the methane is too large to diffuse back out. The methane then gathers at the grain boundaries and can cause the steel to crack.
HTHA is dangerous because, when the carbon and hydrogen combine, decarburization occurs. As the carbon reacts with hydrogen to form methane, the amount of carbon in the steel decreases. The loss of carbon causes the steel to lose strength and its mechanical properties to deteriorate.
The temperatures and pressures that are safe for various types of steel can be determined using a Nelson diagram.
Two commonly used methods for detecting HTHA are the automated ultrasonic backscatter testing and backscatter spectral analysis techniques. Steel that is suffering from HTHA has higher backscatter and lower ultrasonic velocity compared to unaffected steel. It also has a higher S to L-wave velocity ratio.
HTHA can also be detected using magnetic particle testing, time of flight diffraction, or field metallographic replication.
There are many ways in which HTHA can be prevented. The best way is to use high quality steel that can resist the effects of decarburization.
If that’s not possible, then changing the operating conditions until they are they are below the Nelson curve threshold can prevent damage as well. Finally, it’s important to conduct regular inspections to ensure that HTHA hasn’t begun to spread in equipment unseen.