High Temperature Hydrogen Attack (HTHA)

Last update: Jan 13, 2017

High Temperature Hydrogen Attack (HTHA) is an insidious condition that can occur in process equipment exposed to hydrogen at elevated temperatures (at least 400F or 204C), under dry conditions, when hydrogen disassociates into nascent (atomic) hydrogen, which is then driven into the steel by the temperature and pressure of the environment. The atomic hydrogen then reacts with unstable carbides in steel to form methane gas, which accumulates in the microstructural grain boundaries, eventually leading to cracking. This is often hazardous as the equipment usually contains hydrocarbons at high pressures and temperatures.

HTHA is a time-temperature-pressure dependent phenomenon. This means the longer that a piece of equipment is exposed to temperatures and hydrogen partial pressures above its resistance limit, the more damage to the steel will accumulate; and the higher the temperature rises above the limit of the steel, the more rapidly the damage will occur.

Susceptible Areas

HTHA affects carbon and low alloy steels, but is most commonly found in carbon steel and carbon-1/2 Mo steel that is operating above its corresponding Nelson Curve limits. Areas that are hotter, often near the outlet nozzle of catalytic equipment or the inlet nozzle of an exchanger that is cooling the process, are areas of concern for HTHA. Welds often suffer from HTHA degradation as well.

Prevention/Mitigation

Typically HTHA can be avoided by choosing the proper alloy steel or stainless steel cladding to resist the combination of hydrogen partial pressure and temperature, or by adjusting the operating conditions to stay below the Nelson Curve limit for the existing materials of construction. However, there have been several cases where HTHA was found even though operating conditions were below the Nelson Curve.

Inspection Techniques

It can often be difficult to predict the specific areas to inspect for HTHA, since the damage can be very localized. A corrosion or materials specialist, experienced in this particular phenomenon, should be consulted for identifying susceptible equipment, selecting inspection locations, and estimating remaining life of equipment in this service.

Inspection techniques for finding advanced stages of HTHA at the surface include WFMT, MT, and in-situ metallography (e.g., field metallographic replication). Advanced ultrasonic backscatter testing (AUBT) has also been successfully used to find earlier stages of HTHA.

References


Are you interested in learning more about HTHA? Inspectioneering has created an Asset Intelligence Report (AIR) called A Primer on High Temperature Hydrogen Attack. The report is free to download; simply click the button below to get it.

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July/August 2011 Inspectioneering Journal

Tesoro Corporation (NYSE:TSO) today announced the release of the TOP (Triangle of Prevention) Investigation Team Report on the April 2, 2010, incident at the Anacortes Refinery in Washington State. The incident occurred when a heat exchanger in the refinery's Naphtha Hydrotreater unit ruptured, causing an explosion and fire that fatally injured seven employees.

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Marking the one year anniversary of the tragic accident at the Tesoro Refinery in Anacortes, Washington, the U.S. Chemical Safety Board (CSB) released a video safety message.

November/December 2010 Inspectioneering Journal

On April 6, 2010, a tragic accident occurred at the Tesoro Refinery in Anacortes, WA, in the Naphtha Hydrotreater process unit (NHT). During routine operations involving an on-line switching of unit heat exchanger feed trains, seven employees were killed immediately, or died later of thermal burn injuries sustained when a feed-effluent heat exchanger catastrophically failed due to high temperature hydrogen attack (HTHA), releasing a hot, pressurized flammable hydrocarbon/hydrogen mixture which ignited. Tesoro released its investigative results to the media on September 01, 2010.

Avoiding HTHA Failures in Existing Equipment
November/December 2010 Inspectioneering Journal
By John Reynolds at Intertek

High Temperature Hydrogen Attack (HTHA) is a long known and still occurring degradation issue for fixed equipment construction materials in the hydrocarbon process industry where hydroprocess plants (hydrogen plus hydrocarbons) are in service. Though HTHA failures in these units are the focus of this article, it is important to recognize that HTHA damage can also occur in high pressure boiler tubes, hydrogen producing units, synthetic gas units, ammonia plants and other equipment where hydrocarbons may not be involved but high temperatures are involved.

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November/December 2010 Inspectioneering Journal

Per a report in Downstream Today, by Whitney Pipkin Skagit Valley Herald, Mount Vernon, Knight Ridder/Tribune Business News on August 30, 2010, "Tesoro Anacortes Refinery announced Friday what investigators determined was the likely cause of an April 2 blast that killed seven workers."

November/December 2010 Inspectioneering Journal

Understanding, predicting, detecting and controlling high-temperature hydrogen attack (HTHA), have been elusive goals of materials engineers and scientists for over 70 years. The destruction of low alloy steel components exposed to hydrogen through the formation in the microstructure of high-pressure methane bubbles has led to many failures, fires, and sometimes deaths.

January/February 2005 Inspectioneering Journal
By John Reynolds at Intertek

HTHA falls into multiple categories of corrosion mechanisms, including environmentally assisted cracking, hydrogen assisted cracking, and high temperature degradation. Sometimes HTHA is confused with low temperature hydrogen cracking mechanisms that result from hydrogen being driven into steels by aqueous corrosion reactions.

July/August 2004 Inspectioneering Journal
By John Reynolds at Intertek

Decarburization is the antithesis of carburization and rarely results in equipment failure. However, surface decarburization is often a sign that something more serious is going on, ie high temperature hydrogen attack (HTHA), which is well covered in API RP 941, Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants.

November/December 1995 Inspectioneering Journal
By Greg Alvarado at Inspectioneering Journal, and Dr. W. David Wang at Shell Oil Products Company

Nelson Curve changes in the late 1980's provided cause for Shell Oil Company to look at more reliable NDE non-destructive evaluation methods for assessment of materials/equipment in high temperature hydrogen service. The primary change motivating Shell was the lowering of the C-0.5 Mo steel Nelson Curve to the carbon steel level.