What causes corrosion of tubular steel products?
Corrosion is a chemical process that slowly causes the breakdown of a metal. In steel, both oxygen and moisture are needed to begin the corrosion process.
Different steels corrode differently, and at different rates, depending on where they fall on the galvanic chart. Steels that are more anodic, such as mild steel and low alloy steels, are more susceptible to corrosion and corrode more quickly. More cathodic steels, such as 316, 317, and alloy 20 stainless steels, are much more resistant to corrosion.
Aside from the type of steel, there are multiple environmental factors that influence corrosion rate. The amount of moisture, pH level, and pollutants (chlorides, sulphates) can all speed up the rate of corrosion.
Steel is most likely to corrode due to chemicals in the atmosphere, in the soil, in fresh or salt water, or in pipeline applications. The amount and rate of corrosion depends on whether the steel is coated or uncoated. In marine and pipeline applications, the velocity of the fluid around the steel also affects corrosion.
As you can see, there are plenty of factors that affect how much tubular steel corrodes and why. If your application is very harsh or stressful, you will want to start out with a highly resistant alloy (316 or 317) and take further precautions to prevent the metal from breaking down.
Did you know there are multiple ways a metal can corrode? Most people don't. Here are all of the possible corrosion types you can find in tubular steel products.
General corrosion occurs when tubular steel corrodes uniformly across the surface. Certain acids (hydrochloric, sulphuric) are the most common cause of general corrosion. Stainless steel is very resistant to this type of corrosion.
Pitting is a type of localized corrosion that is most common in the presence of chlorides. As the surface of the steel is worn away, it becomes easier for the corrosive particles to penetrate the metal and cause major damage. Pitting corrosion presents as a series of deep OR shallow holes in the metal.
Crevice corrosion occurs when your tubular steel is placed in a tight space, or where there is little room between the tube and another surface. Tight spaces keep oxygen from flowing freely around the metal, which allows corrosive particles to build up. Crevices also occur frequently around assembly points: nuts and washers, screws, bolts, and welds.
Bimetallic/galvanic corrosion is caused by contact between two dissimilar metals (such as steel & zinc or aluminum) in an electrolyte solution (water). The more stable (cathodic) metal will deteriorate more quickly than it would otherwise.
SCC is caused by a mixture of exposure to corrosive particles and physical stress. Bent or welded tubing is especially susceptible. SCC does not usually occur in normal building environments; high-chloride environments, such as marine and offshore applications, should be planned for accordingly.
This type of corrosion isn't very common today, due to continuous progress in metals manufacturing. If the carbon content of steel was too high, it could react with the chromium in stainless steel to produce chromium carbide. The corrosion process was kickstarted with welding, where the heat was high enough to cause the chemical reaction. With the reaction, the level of corrosive-resistant chromium was reduced, leaving the metal vulnerable.
When dealing with corrosion of tubular steel products, you can either prevent it or remove it. This free e-book may help you find a metal finishing service that fits your durability needs: