Determining the Threshold Temperature for Chloride Stress Corrosion Cracking Using a Laboratory Simulation of Field Conditions in Qatar
Authors: Farhat Hanan , Agcaoili John , Fox Jim , Mercier Daniel 
Published: July 2019

Very few oil & gas installation locations around the world experience sustained high humidity and high temperature for long periods of the year as what is experienced in Qatar. For example, the humidity in Qatar readily reaches 90% and the summer temperature, measured on metal surfaces, can reach 80 °C. The severe temperature fluctuations causes seawater to evaporate, then condense, and then dry on hot metal surfaces, thereby enhancing local build-up of aggressive species such as chloride on the metal surfaces. In some situations, this is complicated by sand storms which leave contaminating sand particles on exposed surfaces in this environment.

Qatar has many oil and gas (upstream), petrochemical and chemical plants (downstream) in both offshore and onshore marine locations. For chloride stress corrosion cracking to take place, three important ingredients have to exist: (i) a critical environment, (ii) a susceptible material and (iii) tensile stress. All these three ingredients exist in Qatar onshore and offshore sites. The presence of residual tensile stresses due to welding or other forming process or fit up stresses from assembly increases the susceptibility of the component to CSCC. Chloride stress corrosion cracking can occur fast when evaporation exists even at room temperature. A number of catastrophic CSCC failures of stainless steels roof construction in swimming pool environments has resulted in human causalities over the past decades. Due to the nature of the local environment and the abundance of stainless steel used in Qatari installations the investigation of chloride stress corrosion cracking (CSCC) of stainless steel alloys is very much warranted.

The critical temperature for application of protective coating to prevent CSCC is still to be identified with any certainty. According to NORSOK standard M-001 recommendations, the maximum operating temperature for 316 stainless steel is 60 °C, and for duplex stainless steels is 100 °C, above which, protective coating has to be applied to prevent CSCC. These temperature limits are being questioned and there are concerns being raised about their accuracy when field evidence shows that cracking is occurring at temperatures below these limits. CSCC can lead to oil and gas leaks that have a major impact on the environment and on public safety as well as production loss.

In this research, a modified ISO drop evaporation test was developed to identify the threshold temperature for cracking for three different austenitic stainless steels, mainly 316, 304 and 904L stainless steels. The samples are prone to load and heat, while Qatar specific seawater is dripped on them, the samples are heated using electrical resistant heating. The test is conducted at four different temperatures (room temperature, 40 °C, 50 °C and 60 °C) and under three different loads (70% σ = < /AσΣETHιγηλιγητ>0.2, 80% σ = < /AσΣETHιγηλιγητ>0.2 and 90% σ = < /AσΣETHιγηλιγητ>0.2). The temperature and load are continuously monitored and adjusted when deviated. The lab-built test setup enabled the testing of sixteen parallel fixtures concurrently. The threshold temperature for cracking for the tested material was recorded at each applied load. Severe pitting was observed underneath the salt layer, and was dependent on the applied load. A new threshold temperature for cracking was recorded and a recommendation to the local industry to revise the threshold temperature for chloride stress corrosion cracking is to be issued.