As corrosion most often occurs in aqueous environments, we now explore the different types of degradation a metal can experience in such conditions:. Uniform corrosion is considered an even attack across the surface of a material and is the most common type of corrosion. It is also the most benign as the extent of the attack is relatively easily judged, and the resulting impact on material performance is fairly easily evaluated due to an ability to consistently reproduce and test the phenomenon.
Pitting is one of the most destructive types of corrosion, as it can be hard to predict, detect and characterize. Pitting is a localized form of corrosion, in which either a local anodic point, or more commonly a cathodic point, forms a small corrosion cell with the surrounding normal surface. Pits typically penetrate from the surface downward in a vertical direction. Pitting corrosion can be caused by a local break or damage to the protective oxide film or a protective coating; it can also be caused by non-uniformities in the metal structure itself.
Pitting is dangerous because it can lead to failure of the structure with a relatively low overall loss of metal. Crevice corrosion is also a localized form of corrosion and usually results from a stagnant microenvironment in which there is a difference in the concentration of ions between two areas of a metal. Crevice corrosion occurs in shielded areas such as those under washers, bolt heads, gaskets, etc. These smaller areas allow for a corrosive agent to enter but do not allow enough circulation within, depleting the oxygen content, which prevents re-passivation.
There are numerous types of corrosion that can wreak havoc on oil and gas installations. As you read on, we will focus on two forms of localized corrosion in stainless steel: pitting and crevice corrosion.
Pitting corrosion occurs when the protective oxide layer on the surface of stainless steel breaks down, allowing the bare metal underneath to become susceptible to a loss of electrons through oxidation in the presence of a corrosive aqueous solution. Although usually detectable through thorough visual inspection, these pits can grow deep enough to perforate a tube wall entirely.
Pitting corrosion can also facilitate the initiation of cracks in tensile stressed components. Environments with higher chloride concentrations, including those created by evaporation from deposited saltwater droplets, are prone to pitting corrosion—especially at high temperatures.
When examining metal tubing for pitting corrosion, look for reddish-brown iron oxide deposits as well as potential pits that may have formed on the metal surface. However, rather than occurring in plain sight, crevice corrosion—as its name implies—occurs in crevices. In a typical fluid system, crevices exist between tubing and tube supports or clamps, between adjacent tubing runs, and underneath dirt and deposits that may have accumulated on surfaces.
Crevices are virtually impossible to avoid in tubing installations, and tight crevices pose one of the greatest dangers to stainless steel integrity.
Crevice corrosion occurs when seawater diffuses into a crevice, leading to a chemically aggressive environment where corrosion-causing ions cannot readily diffuse out of the crevice. In such a scenario, the entire surface within the crevice can corrode at a rapid rate. Crevice corrosion can only be observed visually when a tubing clamp is removed from the installed tubing.
Corrosion can, in many circumstances, be minimized by educating workforces with basic materials knowledge:. Material Selection: First, consider the choice of materials for tubing applications, from the tubing itself to tube supports and clamps. Type stainless steel tubing works well in many installations so long as it is kept clean and temperatures are not excessively high. In warmer climates, especially in locations where salt deposits readily form and in installations where rust from carbon steel structural beams and floors accumulate on stainless steel surfaces, corrosion of Type stainless tubing is more readily observed.
For these situations, tubing from superaustenitic e. Pitting Corrosion is the localized corrosion of a metal surface confined to a point or small area, that takes the form of cavities.
Pitting corrosion is one of the most damaging forms of corrosion. Pitting factor is the ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss. The following photo shows pitting corrosion of a SAF duplex stainless steel after exposure to 3. What materials are susceptible to pitting corrosion? Pitting corrosion is usually found on passive metals and alloys such aluminium alloys, stainless steels and stainless alloys when the ultra-thin passive film oxide film is chemically or mechanically damaged and does not immediately re-passivate.
The resulting pits can become wide and shallow or narrow and deep which can rapidly perforate the wall thickness of a metal. ASTM-G46 has a standard visual chart for rating of pitting corrosion. The shape of pitting corrosion can only be identified through metallography where a pitted sample is cross-sectioned and the pit shape, the pit size, and the pit depth of penetration can be determined.
Mechanisms of Pitting Corrosion What causes pitting corrosion? Please use the enquiry form below or the details to the right and one of our company representatives will contact you as soon as possible. Helena St. What is the difference between pitting and crevice corrosion? Corrosion The main difference between pitting and crevice corrosion is the geometry of the corrosion site. How to detect pitting corrosion Pitting corrosion is usually detectable through a thorough visual inspection, with pits able to grow deep enough to perforate a component.
How to detect crevice corrosion With regards to crevice corrosion, crevices exist between joints and underneath dirt and deposits that could have accumulated on surfaces. Got more Questions?
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