Mahmoud T. Shehata

Initiation of Stress Corrosion Cracking and Hydrogen-Induced Cracking in Oil and Gas Line-Pipe Steels

Environmentally assisted cracks in line-pipe steels are initiated either as a result of stresses in combination with environmental effects, as in stress corrosion cracking (SCC), or as a result of trapped hydrogen in the steel, as in hydrogen-induced cracking (HIC). To understand better the mechanism of the crack initiation process, key metallurgical and environmental elements that can affect the cracking phenomena were investigated and are reviewed in this paper. The complexity of both cracking phenomena results from the dependence of SCC and HIC on multiple metallurgical, mechanical, and environmental para- meters that may all influence both crack initiation and propagation; e.g., com- position, microstructure, and nonmetallic inclusions in the steel, applied stress, water chemistry in the field, and ionic concentrations in the groundwater near the pipe surface to name a few for SCC. In addition, for HIC phenomena, one can add the concentration of hydrogen sulfide (H2S) in the fluids transported in the pipe as well as concentration of CO2, pH, etc. In this paper, cracking of line- pipe steels is analyzed critically, with particular attention to the crack initiation process. The paper is divided into two parts: The first part covers SCC and the second covers HIC.

Corrosion Behavior of Die-Cast and Thixocast AXJ530 Magnesium Alloy in Chloride Medium

The corrosion behavior of AXJ530 magnesium alloy in 3.5% NaCl solution at 25°C and pH 7.25 has been studied. Die-cast and thixocast specimens were tested using constant immersion technique, potentiodynamic polarization measurements and electrochemical noise analysis. The microstructure of cast specimens and their surface aspect after corrosion tests were analysed and compared. The results show that thixocast specimens have the best corrosion resistance. Their superior corrosion resistance can be attributed mainly to the presence of large pre-existing α-Mg grains.