The Hendrix Group Reporter©

June 22, 1997 (Vol. VII No. 1) - Revised February 25, 1998

Hydrogen Embrittlement of High Strength Fasteners!

The effects of dissolved hydrogen in the failure of plain and low alloy steels has been studied extensively. Hydrogen embrittlement (HE), in the manifestation of sulfide stress cracking (SSC), is a continuing concern in the oil & gas and petroleum industries, despite years of research into cracking mechanisms, parameters and control methods. Also well documented is hydrogen embrittlement cracking of electroplated fasteners. Less appreciated is the long term effect of hydrogen on cracking of high strength steels and ferrous alloys under atmospheric exposure conditions. This technical note discusses failures of high strength fasteners due to hydrogen embrittlement in atmospheric environments and the danger this cracking presents. Recommendations are provided for reducing fastener failures by HE.

Hydrogen embrittlement is a much studied and written about phenomena. It is the Achilles heel of high strength ferrous steels and alloys. Hydrogen embrittlement, in the manifestation of sulfide stress cracking (SSC), is a continuing concern in the oil & gas and petroleum industries, despite years of research into cracking mechanisms, parameters and control methods. The definitive materials selection document for sour oilfield applications is NACE MR0175. Hydrogen embrittlement during manufacture of electroplated fasteners is also well documented. Less appreciated is the long term effect of hydrogen with cracking of high strength steels and ferrous alloys under atmospheric exposure conditions.

Hydrogen embrittlement is not completely understood; however, it is generally agreed that only atomic hydrogen will enter and diffuse through a steel, in this case during corrosion. Whether absorbed hydrogen causes cracking or not is a complex interaction of material strength, external stresses and temperature. At high strength levels (180 ksi/1241 MPa) only a few ppm of dissolved hydrogen can cause cracking.

We have recently analyzed fasteners where failure was attributed to hydrogen embrittlement cracking. A common denominator in the failures is that the fasteners had been in service many years before failure. Macroscopic evidence of hydrogen embrittlement normally includes a rough, brittle-appearing fracture surface, usually with a single origin, although multiple fracture originscan be present. Fractures typically occur in corroded thread roots. Another characteristic of hydrogen embrittlement cracking is that fractures do not necessarily occur in the last engaged thread as do fatigue fractures. In fact, fracture can occur inside the nut.

SEM examination of fracture surfaces typically show intergranular crack origins, with the final fracture areas exhibiting any combination of intergranular, cleavage, or ductile overload features. Metallographic examination of failed fasteners will normally not show any secondary hydrogen cracks, although subsurface cracking at inclusions has been observed.

The fasteners at most risk of hydrogen embrittlement cracking are high strength studs and bolts, such as those manufactured to ASTM Standards A354 Gr. BD, A490, and socket head capscrews manufactured to ASME/ANSI 18.3 and A574. These standards specify greater maximum allowable hardnesses than other common fastener specifications, i.e., A325, A193, A307 and A449. A193 Gr. B7, a common fastener alloy, does not have a maximum hardness requirement; however, a minimum 1100F (593C) tempering temperature is specified which should result in nominal hardnesses of 25 HRC to 30 HRC. Socket head capscrews have a specified hardness range from approximately HRC 39 to HRC 45 and are typically heat treated to hardness levels greater than 40 HRC. Therefore, they are susceptible to HE at their minimum hardness range.

Fastener hardness is important; atmospheric service fasteners that failed due to hydrogen embrittlement were heat treated to 35 HRC or greater (see Table 1). This hardness is compared with the maximum allowable hardness of 22 HRC specified in MR0175 for most low alloy steels in sour service. HRC 22 is considered conservative for fasteners exposed to non-sour atmospheric service, as the exposure conditions are normally less severe than that of the MR0175 standard test solution. Personal experience field data points are not sufficient at this time to recommend a maximum hardness for fasteners in atmospheric service. However, in one extensive series of laboratory and fields tests (1), the author was unable to fail by HE low alloy fasteners with a converted hardness of HRC 38. Based on the incidence of documented failures, we believe that fasteners manufactured to the specifications A325, A193, A307 and A449 should be resistant to HE cracking in most atmospheric applications. A factor that could increase the potential for HE cracking at lower hardnesses is accelerated hydrogen charging of a fastener acting as the cathode in a galvanic couple.

Fortunately, the number of documented fastener failures in atmospheric service due to HE cracking is not many, considering the number of fasteners in service. A literature review of HE cracking of fasteners in Materials Performance and Corrosion revealed only a few articles, and those were laboratories studies concerned with the effects of cathodic charging and electroplating on HE cracking. HE failure of fasteners may be isolated because fastener corrosion in atmospheric service is usually due to periodic exposure to water, i.e., rain water, cooling tower over spray or firewater. Water from these sources is normally neutral in pH and atomic hydrogen is not readily available at the cathodic sites of corrosion cells. However, if appreciable chlorides are present, as from cooling tower over spray, or wet-dry conditions exist which can concentrate corrosive species in occluded areas of fasteners, i.e., in the thread contact area, low pH solutions can exist, providing a ready source of atomic hydrogen.

The presence of hydrogen at the cathode of a corrosion cell does not necessarily mean that it will automatically diffuse into the steel. Studies have shown that hydrogen diffusion into steel is accelerated by promoters present in some steels, including arsenic, selenium, telurium, antimony and phosphorus, and by cathodic poisons in the aqueous solution, i.e., cyanides. That many corroded high-strength fasteners do not fail is probably because the environments preventing cathodic hydrogen reduction are rare.

Even through reported incidences of fastener failure due to hydrogen embrittlement are low, the consequences of failure can be great in equipment and piping in high pressure, flammable or toxic services. In one incident, two bolts holding the body of a ball valve together failed due to HE, separating the attached piping and releasing a propane cloud. In another instance, seven of twelve body studs in a pump containing high pressure isobutane failed due to HE. Fortunately, no one was injured in either incidence. The risk associated with bolt failures in critical services warrants prudent action to minimize this occurrence.

The following inspection and management practices associated with bolted connections are recommended:

  1. Locate and document all corroded bolted connections during external visual inspections of equipment and piping, especially inspections preceding a scheduled maintenance shutdown. Corroded, high strength fasteners should be replaced during the outage and protected with a barrier coating, anti-seize compound or rust preventative.
  2. Incorporate inspection of bolted connections in risk-based inspection management programs.
  3. Include provisions in maintenance management procedures to protect newly installed fasteners from corrosion.

The role of counterfeit fasteners in fastener failures has received much publicity. A potentially greater hazard in the sudden failure of fasteners due to hydrogen embrittlement is less well appreciated.
(1) Kano, M., Delayed Fracture of High-Strength Bolts - Technical Sub-committee Report, Society of Steel Construction of Japan, In the Transactions of the Iron and Steel Institute of Japan, Vol. 22, 1982, pages 462-477.

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© The Hendrix Group, Inc., 1996
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