Carbon steel is the most common pipe material used in the hydrocarbon processing industry, but the material’s susceptibility to corrosion is well known. Plastic pipes have been investigated as an economical and corrosion-resistant alternative to steel pipe, but this material only works in moderate temperature and pressure environments. Commonly used plastic pipe materials include polyethylene (PE), high density polyethylene (HDPE) and polyvinyl chloride (PVC). To learn more, read the Producing Polymers and Plastics blog, The Use of Plastic Pipe in the Mining and Oil and Gas Industries.
Corrosion can be caused by impurities in the steel pipe, or by impurities in the oil. The 6 Corrosive Components That Can Be Found in Crude Oil is an article on Corrosionpedia that describes impurities often found in crude oil that could cause corrosion in pipelines, vessels and refinery equipment. These include:
Brackish water, which is drawn from crude oil wells along with hydrocarbons. During preheating, these chloride salts can break down into HCl which when cooled to temperatures lower than the dew point of water, reacts with condensing water to produce extremely corrosive hydrochloric acid.
Carbon dioxide may be trapped in oil reservoirs or present in crude oil as a by-product, but usually it’s injected into crude oil wells for enhanced oil recovery. CO2 causes sweet corrosion, which typically results in pitting or material loss. Sweet corrosion can be eliminated by adding inhibitors to the crude oil or by using stainless steel pipe.
Naphthenic acid is an organic acids sometimes present in crude oil. One of the most common ways to reduce naphthenic acid corrosion (NAC) in crude oil refining systems is by blending a high Total Acid Number (TAN) crude oil with a crude oil having low TAN. Injecting corrosion inhibitors into the crude oil stream is another method.
Sulfidation. Crude oils usually contain sulfides that can cause corrosion at high temperatures. Sulfidation can be controlled by selecting the proper steel using “McConomy” curves.
Bacteria can cause corrosion in oil and gas storage and transportation facilities and can be eliminated by adding biocides.
This post focuses on the many causes of steel pipe corrosion resulting from impurities in the oil. Several Analyzing Metals posts discuss impurities in the steel pipe as an equally troublesome source of potential problems. Sulfur is an impurity in steel that can result in sulfidic corrosion in the right conditions. Carbon steels with low silicon (<0.10%) content have been shown to corrode at an accelerated rate when exposed to sulfidation corrosion conditions. To learn how handheld x-ray fluorescence (XRF) technology can consistently detect silicon levels in steel at a level of 0.03% or less, read about the analysis of silicon content in steel and sulfidic corrosion, including surface preparation, correlation curve to lab certified results, and repeatability testing.
Flow-accelerated corrosion occurs when carbon steel piping and components are degraded by flowing water or steam water with low-dissolved oxygen. Handheld XRF is a valid method for monitoring the trace alloy content of carbon steel piping where FAC is a concern. Read the application note, Use of Handheld XRF in the FAC Inspection Protocol to see data and analysis of the performance of a handheld XRF analyzer for the FAC application.