Corrosion

NACE:

• Stress Corrosion Cracking

- Stress corrosion maybe defined as the combined action of sustained tensile stress and corrosion that may lead to the premature failure of materials. Certain materials are more susceptible than others. If a susceptible material is placed in a corrosive environment under tension of sufficient magnitude, and the duration of service is sufficient to permit the initiation and growth of cracks, failure will occur at a stress lower than that which the material would normally be expected to withstand. The corrosive environment need not be severe in terms of general corrosive attack.

- Service failures due to stress corrosion are frequently encountered in cases where the surfaces of the failed parts are not visibly corroded in a general sense.

- If failure is to be avoided, the total tensile strength in service must be maintained at a safe level. There is no absolute threshold stress for stress corrosion, but comparative stress-corrosion thresholds can be determined for materials subjected to controlled conditions of test. Estimates of the stress-corrosion threshold for a specific service application must be determined for each alloy and heat treatment, using a test piece, stressing procedure and corrosive environment that are appropriate for the intended service.

SSC Reference to NACE MRO 175

Corrosion is a dominant contributing factor to failures and leaks in pipelines. To deal with this issue, the CAPP Pipeline Technical Committee has developed industry recommended practices to improve and maintain the mechanical integrity of upstream pipelines. They are intended to assist upstream oil and gas producers in recognizing the conditions that contribute to pipeline corrosion incidents, and identify effective measures that can be taken to reduce the likelihood of corrosion incidents. This document addresses design, maintenance and operating considerations for the mitigation of internal corrosion in sour gas pipeline systems constructed with carbon steel materials. Within this document, sour gas corrosion could be expected to occur when: · H2S concentration in the gas phase is greater than 500 ppm (These limits are supplied as a guideline only and may not be absolute) · H2S is dissolved in free water This document does not address failures due to environmental cracking such as sulphide stress cracking (SSC) and hydrogen induced cracking (HIC). This document also does not address gas gathering systems fabricated with aluminum and non-metallic materials.

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