Technical Article
Pitting Corrosion vs . Crevice Corrosion : Identifying and Preventing
Preventing pitting and crevice corrosion can save downtime and money . Identifying the type of corrosion affecting a system is the first step to eliminating it in the future .
By Michael Bestic , Senior Materials Engineer , Swagelok
Corrosion can occur on nearly any metal used in offshore and nearshore applications . Failure caused by corrosion is not inevitable and can cost companies billions of dollars each year . Companies who understand what type of corrosion is damaging their tubing systems can preemptively take precautions to reduce corrosion risks . Identifying the type of corrosion requires an intimate understanding of what causes specific types of corrosion . Exploring where to find corrosion is worth it , so risks can be minimized on oil rigs and refineries to help prevent significant profit loss and extended downtime .
How Corrosion Occurs
Corrosion , the loss of material on the metal surface , occurs when materials interact with the environment and results in a reduction in the wall thickness of components . This is especially true for components made from carbon or low alloy steels , making those components more susceptible to corrosion-related mechanical failure .
It is common to see metallic tubing systems in analytical and process instrumentation , hydraulic lines , and control and utility applications . The material most often used is stainless steel , containing at least 16 % chromium . Chromium creates an oxide layer that offers protection to the metals from corrosion , though stainless steel corrosion can still occur . In offshore and nearshore applications , environmental conditions can cause the protective oxide layer to break down and leave even stainless steel components vulnerable .
Identifying Common Types of Corrosion
Though corrosion comes in many forms and can destroy oil & gas installations if not well managed , it is important to
Figure 1 . Localized pitting corrosion happens due to electrochemical reactions that initiate the formation of small cavities within a surface . © 2022 Swagelok Company .
choose the right material for the operating environment . Even when the proper material is chosen , however , localized corrosion can still happen . Identifying whether a system suffers from pitting corrosion or crevice corrosion is critical to create an effective plan to combat it .
Pitting Corrosion
When the protective oxide layer on a stainless steel component is damaged by contact with a corrosive aqueous solution , the electrochemical reaction causes the unprotected metal to lose electrons through oxidation . The resulting electrochemical reaction causes small cavities to form on the metal , commonly known as “ pits ” ( Figure 1 ).
Typically , pitting corrosion can be seen on visual inspection , but the extent of the pits may not be initially obvious . Sometimes , the pits are so deep that they perforate the material in the components . In addition , pitting corrosion can make it easier for cracks to form in tensile-stressed components . Environments with high chloride concentrations , like oil & gas rigs surrounded by saltwater , can often create the perfect conditions for pitting corrosion of stainless steel material , particularly if the components are subject to high temperatures . Pitting corrosion can be detected by the visual cue of reddish-brown iron oxide deposits and the presence of pits that may have formed on the metal surface .
Crevice Corrosion
Like pitting corrosion , crevice corrosion ( Figure 2 ) occurs when the protective oxide film on a component is broken . Crevice corrosion differentiates itself from pitting corrosion by occurring in crevices rather than exposed metal surfaces .
In most fluid systems , crevices are formed between tubing and tube supports or clamps , between adjacent tubing runs , and under dirt and deposits that naturally accumulate on the surfaces of components . They are virtually unavoidable during tubing installations and pose significant danger to stainless steel integrity . Once seawater diffuses into a crevice , there is often nowhere else for it to go . It remains trapped in the crevice and creates a chemically aggressive environment where crevice corrosion can occur ( Figure 3 ). Since there is no way for the seawater to exit the crevice , corrosion can spread throughout the crevice ’ s surface , often quite quickly .
Unfortunately , crevice corrosion is typically only discovered when a tubing clamp is removed from the tubing . It is also crucial to understand that crevice corrosion can occur at lower tempera-
8 Stainless Steel World Americas - August 2022 | www . ssw-americas . com