51315-6050-Conservatism in Sour Testing with Hydrogen Sulfide Partial Pressure Exposures—Towards a More Consistent Approach

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Product Number: 51315-6050-SG

ISBN: 6050 2015 CP

Author: Sytze Huizinga

Publication Date: 2015

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Sour stress cracking - either sulfide stress cracking (SSC) or environmental stress corrosion cracking (SCC) - has long been recognized to result from a combination of three principal elements – a susceptible material stress and a sour exposure. Generally industry testing using the standard test methods has been successful in qualifying both carbon steel and corrosion resistant alloy (CRAs) materials for field exposures by using equivalent partial pressures in lab tests. Yet the materials tested are rarely of the combination of the “hardest” most “lean” alloy and least “susceptible” microstructure one may encounter compared to the broader range of materials delivered to the field. Likewise stress while generally higher in controlled test exposures compared to “typical” average field exposures are only slightly if at all conservative compared to a multitude of local stress riser conditions one may encounter in field equipment. So this raises the question – why the good correlation between lab qualification of materials and field experience?An examination of the actual non-ideal behavior of sour gas suggests that practical low pressure laboratory testing may be inherently several-fold more severe than the field exposure when equivalent partial pressures are used for material qualification – an degree of conservatism implied by ISO 15156 when partial pressures are used. For room temperature tests simulating a well exposure this conservatism may be 3 to 4 fold above the field exposure assuming fugacity a true gas property is controlling. However this exposure conservatism becomes less at typical pressures employed in surface facilities or when simulating hotter high pressure well environments for CRAs identifying that the industry approach is inconsistent in the degree of conservatism it employs. This in turn suggests that the reality of testing and qualification of materials using partial pressure likely represents a widely varying degree of conservatism and is in need of a more consistent approach.This paper examines the impact of material stress and exposure factors on sour corrosion cracking testing and suggests a semi quantitative approach to achieve adequate conservatism.Keywords: sour testing SSC SCC H2S partial pressure fugacity conservatism