External chloride stress corrosion cracking of stainless steel lokring pipe connectors

Health and Safety Executive - Safety alert

Introduction:

This Safety Alert highlights the issue of chloride stress corrosion cracking which has been found to affect stainless steel (grade 316/316L) Lokring pipe connectors. The cracking has occurred in the driver / swage ring section of the connector as shown:

Background:

Dutyholders have found cracks in the driver ring section of stainless steel Lokring pipe connectors. This component is subject to tensile hoop stress and when exposed to a chloride-containing environment (such as marine and coastal areas or saltwater deluge systems) can be affected by chloride stress corrosion cracking

All the cracks found to date have been in unpainted stainless steel pipe connectors. However, the crevice in the connector (as shown above) presents a potential initiation site by localised corrosion, so painting may not guarantee elimination of the problem.

Visual inspection and dye penetrant inspection can only find cracks that are surface breaking. Radiography has identified internal cracks that are not yet visible. Crack propagation and failure could therefore occur even where no cracks have been seen at the surface.

The driver ring is essential in maintaining the integrity of the connector. Cracking of this ring therefore presents a serious loss of containment risk.

HSE research report RR902 Chloride stress corrosion cracking in austenitic stainless steel describes the process of chloride stress corrosion cracking in austenitic stainless steel and how to assess susceptibility to this degradation mechanism.

Action required:

• Dutyholders should identify where stainless steel Lokring connectors are in service in a chloride-containing environment, and remove from service where failure could present a safety risk.
• In order to reduce the risk in the most timely manner, the removal of connectors should be prioritised by the consequence of failure, time in service and most corrosive environment (eg temperature, presence of chlorides etc) as described in RR902.

Relevant legal documents:

The relevant legal requirements are:

1. The Health and Safety at Work etc Act, 1974, Sections 2 and 3.
2. Offshore Installations (Prevention of Fire & Explosion, and Emergency Response) Regulations 1995, Regulation 9
3. Provision & Use of Work Equipment Regulations, 1998, Regulation 5.
4. Control of Major Accident Hazard Regulations, 1999, Regulation 4.

References:

RR902 Chloride stress corrosion cracking in austenitic stainless steel

Further information:

Health & Safety Executive
HID Energy Division (Offshore)
Mechanical Engineering
Redgrave Court
Bootle L20 7HS