Managing and mitigating piping corrosion is paramount to the integrity of critical infrastructure. There are 2.7 million miles of pipelines managed by the Pipeline and Hazardous Materials Safety Administration (PHMSA), a subdivision of the United States Department of Transportation. PHMSA is responsible for routine inspections, regulations and enforcing preventive maintenance operations. Founded in 2004, PHMSA has received additional funding with a budget that has more than doubled since 2007 and investments that have paid dividends as they have been successful in decreasing the number of serious pipeline incidents by 20 percent since 2009.
Forms of Pipeline Corrosion
Pipelines managed by the PHMSA transport crude oil, liquefied natural gas (LNG) and hazardous liquids. They are predominantly composed of steel, but there are numerous forms of piping corrosion that can affect operations, including uniform corrosion, pitting corrosion, galvanic corrosion, crevice corrosion, selective corrosion, intergranular corrosion and microbiologically influenced corrosion.
Uniform corrosion, the most common type of corrosion, propagates uniformly across exposed surfaces. Untreated surfaces remain susceptible to uniform corrosion in the presence of corrosive species which can rapidly deteriorate metallic surfaces due to either anodic or cathodic reaction. Oxidation in the presence of air or water is the most common cause of uniform corrosion, which is easily identified by rust. Popular methods used to mitigate uniform corrosion include the use of corrosion inhibitors, protective coatings and cathodic protection.
Pitting corrosion is the localization of corrosion to small areas or activation sites. It occurs along treated surfaces where abrasions or inclusions in the base material exist, or when the surface isn’t treated properly to withstand environmental effects or exposed to elevated concentrations of aggressive chemical species. There are numerous methods that can lead to pitting corrosion, and a material's resistance to pitting corrosion is drastically reduced when temperatures exceed the critical pitting temperature (CPT) defined by ASTM Standard G48-03. Proper material selection, pH control, and cathodic or anodic protection are used to mitigate pitting corrosion.
Galvanic corrosion, also referred to as bimetallic corrosion, occurs when there is a joint between two metallic or semi-metallic conductors with contrasting electrochemical potentials when exposed to an electrolytic fluid. The electrochemical potential is specific to the environmental conditions and electrolyte present. The difference in potential is the driving force for galvanic corrosion as current flows through the electrolyte towards the more noble metal, while the less noble metal or active metal undergoes galvanic corrosion. To mitigate galvanic corrosion, avoid threaded joints between materials that are far apart in the galvanic series, insulate dissimilar metals whenever possible and ensure coatings are applied and maintained for integrity.
Crevice corrosion is a localized area of corrosion that occurs at or immediately adjacent to a joint. The gap or crevice between the joined material acts as an electrochemical concentration cell. Chlorides concentrate in the crevice, which becomes depleted in oxygen and exhibits a lower pH, becoming the anode of an electrochemical reaction. Crevice geometry, material composition and environmental conditions heavily influence crevice corrosion. ASTM Standard G48-03 specifies a critical crevice temperature (CCT), which is the minimal temperature known to initiate crevice corrosion; this is typically much lower than the CPT. Use of welded butt joints, solid non-absorbent gaskets and alloys with an increased resistance to crevice corrosion helps mitigate crevice corrosion. Care should also be taken to eliminate crevices in lap joints by continuous welding or soldering.
Dealloying or selective corrosion is a process where one or more components in a solid solution are either replaced or lost through electrochemical interactions. Examples of dealloying include decarburization, decobaltification, denickelification, dezincification and graphitic corrosion. In each of these cases, the electrochemical potential of the alloying elements is the driving force behind selective corrosion. Prevent dealloying by integrating sacrificial anodes, cathodic protection or by selecting an alloy with a higher resistance to dealloying.
Intergranular corrosion or interdendritic corrosion occurs along grain boundaries. Compositional variances are a leading cause of intergranular corrosion, and it is more prevalent in alloy castings. It can also lead to crack propagation in the presence of moderate tensile stresses. Use low carbon stainless steels, titanium and niobium alloys, or post-weld heat treatments to prevent intergranular corrosion.
Microbiologically influenced corrosion (MIC) occurs when either aerobic or anaerobic bacteria leads to accelerated rates of corrosion. Sulfate-reducing bacteria (SRB) is a common anaerobic bacteria that is responsible for most instances of accelerated corrosion experienced in offshore steel structures and marine environments. To prevent MIC, regular pigging or cleaning operations should be performed on pipes that are predisposed to elevated levels of sulfides. Chemical treatments or inhibitors such as biocides can also be used to control the population of bacteria in a fluid.
Preventing pipeline corrosion is paramount, yet much less than a straightforward procedure. To ensure your preventive maintenance procedures yield a higher return on investment, careful consideration should be given to material requirements and treatments methods. Preventive maintenance solution should be tailored to best suit given process fluids and environmental conditions.