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HTHA in Base Material

HTHA along HAZ

HTHA | High Temperature Hydrogen Att ack

High temperature hydrogen att ack (HTHA) can occur when steel is exposed to atomic hydro-gen (H) at elevated temperatures and pres-sures. Under these conditi ons, some hydrogen molecules (H2) break apart into individual hy-drogen atoms (H). Individual hydrogen atoms can diff use into steel at high temperatures and react with carbon (C) in the steel, formingmethane (CH4). Because methane molecules are too large to diff use through steel, the methane accumulates, forming extremely high pressure bubbles which connect to create mi-cro-fi ssures at grain boundaries. In advanced stages of HTHA, fi elds of micro-fi ssures con-nect to form cracks. The chemical combinati on of carbon and hydrogen results in decarburiza-ti on of the steel. Decarburizati on is the loss of carbon from the steel, reducing the strength of the metal.

Industry codes and standards rely on data compiled in the American Petroleum Insti tute (API) Recommended Practi ce 941 to deter-mine combinati ons of hydrogen parti al pres-sures and temperatures for which HTHA is not expected to occur (See chart at Top). The Nel-son curves were developed from industry ex-perience and have been adjusted over ti me to

refl ect new reports of HTHA. For a given type of steel, combinati ons of hydrogen parti al pressure and temperature below the Nelson curve are considered safe.

Since several parameters infl uence a mate-rial’s suscepti bility to HTHA, our Asset Integ-rity Management group (AIMS) reviews & coordinates eff orts with your process team to develop a comprehensive inspecti on plan that identi fi es equipment with the highest risk, thereby reducing the danger of any cata-strophic events. The planning work can be done well in advance of a turnaround, so that all necessary factors are considered.

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MISTRAS uti lizes several NDE techniques to determine the presence and the severity of HTHA. The primary inspecti on techniques are the Advanced Ultrasonic Backscatt er Tech-nique and Advanced Backscatt er Spectral Analysis. These techniques use proven ultra-sonic technologies that evaluate the frequency dependence, velociti es and several other fac-tors to help determine which assets may have been aff ected by HTHA. Further investi gati on involves the use of metallographic replicati ons to evaluate the microstructure as well as the other material properti es.