Questions about wet hydrogen sulfide corrosion

My understanding: The higher the carbon content, the greater the tendency to intergranular corrosion. Wet hydrogen sulfide, a strongly reducing medium, accelerates the rate of corresponding corrosion.

The higher the carbon content, the greater the strength, but the more severe the corrosion. Carbon is an element that strongly promotes the formation and stabilization of austenite as well as the expansion of the austenite region in austenitic stainless steels. It is considered a harmful element in such steels, because when welding or heating to 450–850 degrees Celsius, carbon can combine with chromium in the steel to form CR23C6, resulting in localized chromium depletion and a decrease in the steel’s resistance to intergranular corrosion. Wet hydrogen sulfide accelerates the corrosion rate of materials, and stress corrosion also occurs in austenitic steels.

The two people upstairs really dare to speak up – they mix wet hydrogen sulfide stress corrosion cracking with intergranular corrosion... The person on floor 3 also said that wet hydrogen sulfide SCC affects stainless steel sensitively; I feel a lot of pressure these days at Haichuan. The factors affecting cracking in a wet hydrogen sulfide environment include pH value, material hardness, inclusions in the steel, temperature, and the effects of heat treatment. These are further divided into HIC, SOHIC, and SSCC. The influencing factors are different as well. Let’s take a look at NACE-0175 “Metallic Materials for Oilfield Equipment Resistant to Sulfide Stress Cracking” first.

This post was last edited by penryn on 2011-7-29 09:12. It involves both hydrogen corrosion and sulfide corrosion, as a galvanic cell is formed in a wet hydrogen sulfide environment; at the cathode, ionization produces H+, which reacts with the metal to form H2. It penetrates into the metal, generating internal pressure and creating localized high pressures); then stress concentration occurs, leading to the formation of numerous microcracks (HIC – Hydrogen-Induced Cracking) and bubbles (HB – Hydrogen Bubbling). As a result, both toughness and strength decrease, and the cracks expand further under stress (SOHIC – Stress-Induced Hydrogen-Induced Cracking). The anode produces metal sulfides. The ratio of the volume of compounds such as FeS, NiS, MnS, CrS, CuS, etc. to the volume of the base metal is 2.5–3.0; the resulting sulfide films have higher internal stresses, which can cause the films to crack (sulfide stress corrosion cracking, SSCC). :)

An increase in carbon content enhances the material’s susceptibility to stress corrosion in sulfides

Reply to 5# penryn: I think that by limiting the carbon equivalent, not only is the carbon content restricted, but also the manganese content. In a wet hydrogen sulfide environment, the sulfides formed should be bar-shaped impurities such as MnS. This is just my personal opinion; I look forward to everyone’s feedback

That makes sense. Below are the materials I found regarding hydrogen sulfide corrosion. Alloying elements and heat treatment: Harmful elements: Ni, Mn, S, P; Beneficial elements: Cr, Ti. Carbon (C): Increasing the carbon content in steel raises its susceptibility to stress corrosion cracking in sulfides. Nickel (Ni): Increasing the nickel content in low-alloy steel reduces its resistance to stress corrosion cracking in solutions containing hydrogen sulfide. The reason is the increase in nickel content, which may lead to the formation of a martensite phase. Therefore, the nickel content in steel should not exceed 1%, even when its hardness HRC is <22. Nickel-containing steels have a higher tendency to stress corrosion cracking because nickel has a significant influence on the cathodic process. The lowest cathodic overpotential can be observed in nickel-containing steels; as a result, the absorption of hydrogen by these steels increases, leading to an enhanced tendency for metal stress corrosion cracking. Chromium (Cr): It is generally considered that for steels used in hydrogen sulfide-containing solutions, a chromium content of 0.5% to 13% is fully feasible, as they can achieve a stable microstructure after heat treatment. Regardless of the chromium content, no difference in stability was observed among the tested steels. Some authors in the literature also believe that a high chromium content is advantageous, arguing that the presence of chromium facilitates the passivation of steel. It should be noted, however, that this effect only occurs when the chromium content is greater than 11%. Molybdenum (Mo): When the molybdenum content is ≤3%, it has little effect on the load-bearing capacity of steel in a hydrogen sulfide environment. Titanium (Ti): Titanium’s effect on the stress corrosion cracking sensitivity of low-alloy steels is similar to that of molybdenum. Tests have shown that in a hydrogen sulfide environment, adding titanium (0.09% Ti) to steel with a low carbon content (0.04%) improves its stability to some extent. Manganese (Mn): Manganese is an element that tends to segregate, and it is very important to study its role in the process of corrosion cracking caused by sulfides. Once the Mn and C contents in the segregated zone reach a certain ratio, a martensite/bainite microstructure with high strength but low toughness is formed during steel production and equipment welding; this structure exhibits very high hardness, which is detrimental to the equipment’s resistance to SSCC. For carbon steel, the manganese content is generally restricted to less than 1.6%. A small amount of Mn can convert sulfur into sulfides, which are then removed from the steel. Additionally, when deoxidizing steel, the use of a small amount of manganese helps to form a favorable microstructure, thereby having a positive effect. In the oil industry, steel with a high manganese content is commonly used for manufacturing oil pipes and casings, such as China’s 36Mn2Si steel. (Increasing hardness) Sulfur (S): Sulfur is detrimental to the stress corrosion cracking resistance of steel. As the sulfur content increases, the stability of the steel deteriorates sharply, mainly because sulfide inclusions act as sites for hydrogen accumulation, causing the metal to develop a defective structure. At the same time, sulfur is also a promoter for hydrogen adsorption. Therefore, the reduction, dispersion, and spheroidization of non-metallic inclusions, especially sulfides, can improve the stability of steel (particularly high-strength steel) in media that cause metal hydrogenation. Phosphorus (P): In addition to forming fusible eutectic inclusions that can cause red brittleness (thermal brittleness) and reduced plasticity in steel, it also inhibits the recombination process of hydrogen atoms (Had + Had → H2↑), thereby increasing the tendency of the metal to absorb hydrogen; this in turn reduces the stability of steel in acidic environments containing hydrogen sulfide.