What is the function of mercaptans present in crude oil in corrosion?

Mercaptans, also known as thiols, are organic sulfur compounds present in crude oil, characterized by the presence of a sulfhydryl group (-SH) bonded to a carbon atom. They contribute to corrosion through several mechanisms, primarily by directly attacking metal surfaces and accelerating electrochemical corrosion processes. One primary way mercaptans contribute to corrosion is through direct chemical attack. The sulfhydryl group (-SH) in mercaptans is reactive and can directly react with metals like iron to form metal sulfides and release hydrogen. This process weakens the metal structure and leads to corrosion. The reaction is typically accelerated at higher temperatures. The presence of mercaptans can also increase the rate of electrochemical corrosion. They can act as cathodic depolarizers, which means they facilitate the reduction reaction at the cathode, accelerating the overall corrosion process. In the presence of water, mercaptans can dissociate to form mercaptide ions (RS-), which are highly corrosive. These ions can attack the metal surface, leading to pitting corrosion. Mercaptans can also interfere with the formation of protective oxide layers on metal surfaces. These oxide layers normally provide some degree of corrosion resistance, but mercaptans can prevent their formation or disrupt existing layers, making the metal more vulnerable to corrosion. Furthermore, the presence of mercaptans can exacerbate other forms of corrosion, such as sulfide stress cracking (SSC), a type of hydrogen-induced cracking that occurs in high-strength steels exposed to hydrogen sulfide (H2S). Mercaptans can increase the amount of atomic hydrogen absorbed by the metal, promoting SSC. The corrosivity of mercaptans depends on their concentration, molecular structure, temperature, and the presence of other corrosive agents. Smaller mercaptans are often more corrosive than larger ones. Therefore, the presence of mercaptans in crude oil is a significant concern for corrosion in refineries and pipelines.