Microalloyed pipeline steels have been developed for applications in hydrogen sulfide environments (sour environments), focusing on the required yield strength. Nevertheless, the cracking susceptibility is mainly affected by the microstructure produced during the manufacturing processes. In sour environments, most of the failures can be related to the susceptibility to hydrogen embrittlement for higher mechanical grades. However, sulfide failures in lower grade steels are related to the anodic dissolution. In this work, APi 5L X52 steels were evaluated in order to determine their cracking susceptibility in pressurized sour environments with brine solutions at 50°C, by using M-WOL specimens. The steels showed a marked corrosion in the surfaces exposed to the environments showing dissolution bulbs that may be related to slip-dissolution as the main mechanism in order to induce cracking.
In general with high strength and stainless steels sulfide stress corrosion cracking (SSCC) has been recognized as a serious problem for the petroleum and petrochemical industries. Also similar failures have been reported in medium strength microalloyed pipeline steels. Commonly, the sour environments contain amounts of salt water, hydrogen sulfide (H2S), and carbon dioxide. Upon its composition and temperature, this environments, may cause general corrosion, localized corrosion, and stress corrosion cracking in the materials used. A variety of variables contributed to develop SSCC in iron base alloys. These variables can be classified in two categories: those concerned to environmental effects such as chloride concentration, hydrogen sulphide concentration (i.e. partial pressure), temperature and applied potential; and those related to the mechanical and metallurgical variables such as hardness, strength level, alloy composition, microstructure, heat treatment, cold work, and surface condition.
In sour environments when it comes in contact with water the H2S can dissociates in ions according to:
H2S ~ HS+ H ÷ and H2S --~ 2H ÷ + S 2- (1)
Then a corrosion reaction occurs with steel thus forming ferrous ion at anodic sites and reduction of hydrogen at cathodic sites at the steel surface, according to the following reactions:
9 Fe ~ Fe 2÷ + 2e- (anodic site) (2) HS + e- ~ H ° + S 2 (cathodic site) (3) xFe2÷+yHS(aq) ~ FexSy + yH ÷ (overall reaction) (4)
Because of the presence of dissociated hydrogen sulfide the recombination of atomic hydrogen to produce molecular hydrogen is inhibited. Then the penetration of hydrogen into the steel is facilitated, intensifying any embrittlement effects.
An approach to elucidate the mechanisms of the corrosion and cracking process is to study the evolution of these processes over time. Corrosive agents such as H2S dissolved in brine solutions (3%NaC!), with low pHs, are frequently used for this purpose. These solutions have been employed in order to evaluate steel corrosion and chemical composition of the protective films produced, their thickness and evolution over the immersion time in the interfaces where corrosion products and sour environment interact. ~0.~ As a result of such investigation two stages have been distinguished on the general corrosion mechanism. At first stage is the charge transfer process, that take place in the metal corrosion-products interface, and second stage is a transport process through of the protective sulfide film. The transport process is the limiting stage in the overall corrosion process.
