Version No:

1

OG Status:

Fully Open

Author Section:

HID CI 1G

Issue Date:

20 March 2009

Review Date:

20 March 2011

Target audience:

HID discipline specialists (mechanical engineering), HID regulatory specialists, FOD specialist group (mechanical engineering)

Purpose

The purpose of this circular is to raise the awareness of the possible degradation of cold formed austenitic stainless steel fasteners in contact with chlorides at ambient temperatures.

Aim

The aim of this SPC is to provide mechanical engineering inspectors with information for use when inspecting plants that contain substances that could include chlorides.

Regulatory specialists are not expected to attempt a detailed technical inspection using this SPC. It may be useful, however, in supporting interventions associated with the HID chemical industry strategy – particularly key goal 1 ‘Prevention of major accidents’.

Background

HSE has investigated an incident where the failure of cold formed 304 stainless steel screws in contact with a hastelloy flange at ambient temperatures has lead to a significant release of phosphorous oxychloride. Other stainless steel fasteners on the plant were examined and found to be suffering from stress corrosion cracking. New stainless steel fasteners were examined after six months service and further corrosion spots were found; with one bolt showing clear stress corrosion cracking.

During the investigation it was found that a similar failure had occurred on a hydrochloric acid system at another plant. It was also noted that chlorine induced stress corrosion cracking of highly stressed austenitic stainless steel at temperatures of around 30C has been widely reported in swimming pool environments - see HSE guidance note SIM 05/5002/18 [25.2KB]

Degradation mechanism

Very low levels of chloride in the atmosphere, or leaking across a nominally sealed flange connection, can collect in crevices and become concentrated over time. Such crevices could be the underside of a bolt head or the root of a screw thread. Where the chloride concentration coincides with a highly stressed 304 type stainless steel then a risk of stress corrosion pitting and cracking will exist at ambient temperatures. Cold formed fasteners retain very high residue stresses. Therefore in a chloride environment there will be sufficient stress for stress corrosion cracking to be initiated in cold formed 304 stainless steel.

Where a stainless steel fastener is in contact with a dissimilar metal, e.g. hastelloy, then the potential difference between the two metals may act to accelerate the chloride induced corrosion.

Recommendations

Where the failure of cold formed 304 stainless steel fasteners could lead to a loss of containment of a hazardous substance or the unintentional release of stored energy the operator should:

1. Ensure that there are no chlorides in close proximity to the fasteners – either within the containment system or in the atmosphere local to the fixing.
2. If chlorides are present then they should consider the use of a different fastener material that is not susceptible to stress corrosion cracking.

Further Information

For further information contact HID CI 1G (VPN 520 2148)