Hydrogen Damage and Cracking

With ductile metals, such as low carbon steel, the material can easily deform and the reduction in strength isn’t terribly significant. Figure 3 shows a piece of SAE 1020 steel with a series of blisters that are about a quarter inch in diameter and are filled with hydrogen. It was part of some chemical process equipment and wasn’t highly stressed so the blisters didn’t cause any problems, but larger blisters can cause reduced ductility.

However, as steel is processed to become stronger, it almost always becomes less ductile and the effect of the atomic hydrogen becomes more critical. Figure 4 shows the magnified fracture of a steel spring used in a piece of pharmaceutical equipment. The bottom of this HRC 50 spring was rusted, the result of the cleaning process and a poor machine design. Looking at the crack face, the corrosion at the origin is readily visible, and a metallurgical analysis confirms that hydrogen was involved.

We know that hydrogen cracking is the result of a combination of metal chemistry, temperature, stress and time. Nevertheless, the only way for sure to know if hydrogen has contributed to a failure is to have the part analyzed for hydrogen content, but there is no hard and fast rule as to the effect that corrosion has on causing the failures.

One of my first lessons about the effect of corrosion involved a $200,000 roller bearing in one of our machines. This was before the era of predictive maintenance, and we had removed it from position and opened it up to inspect the condition of the contact surfaces. The cause of the problem was that, after removing it from a very dirty environment, we had washed it with hot water that managed to get past the seals. So, when we opened the bearing there were a series of black marks on the inner and outer rings, and Figure 5 is a microscope’s view of the marks. They are corrosion pits and the technical staff from the bearing manufacturer told us that the hydrogen from the pitting had reduced the L10 life of the bearing to 10–20 percent of the original.

Black marks on the contact path of a rolling element bearing are almost always symptoms of corrosion that will greatly reduce ball and roller bearing life. With similar stress mechanisms and hardened steels, corrosion will also reduce the life of gear teeth and chain components.