Corrosion: Pervasive, Problematic, Preventable
Corrosion—the deterioration of engineering materials, most commonly metals, by chemical interaction with their environment—is an extraordinarily costly problem. And much of that damage is preventable, if only companies knew what to look for and took simple prevention measures.
Just about every metal used to build our world corrodes, but only under certain circumstances. There are steps to prevent corrosion in oil and gas applications, particularly offshore, and these require a fundamental understanding of corrosion and what causes it. A working ability among equipment and system operators to visually identify corrosion before it causes a bigger problem—and the ability to know where to look for hidden corrosion—can minimize risks on rigs and in refineries, saving significant time and money in many cases.
Identifying Common Types of Corrosion
Stainless steel tubing is one of many areas where corrosion prevention can make a difference. Tubing systems are used for process and analytical instrumentation, hydraulic lines, and control and utility applications. The mechanical failure associated with corrosion—the escape of fluid or chemicals from tubing—can result in an unsafe work environment, the release of harmful emissions, and in worst case scenarios, a situation that leads to a catastrophic system failure. Primarily, two forms of corrosion affect stainless steel tubing: pitting and crevice corrosion.
Pitting of a metal occurs when its protective layer, e.g., a thin chromium-rich oxide film on stainless steels and nickel alloys, breaks down, allowing the exposed metal atoms to give up their electrons easily, resulting in corrosion. This electrochemical reaction initiates the formation of tiny pits. Corrosion accumulates in the pits, leading to conditions which cause these pits to grow deeper, eventually even penetrating a tube wall entirely. Pits can also lead to cracks in stressed components. Environments with higher chloride concentrations, including those created by evaporation of water from deposited salt water droplets, are more prone to cause pitting corrosion, especially at higher temperatures, because these conditions help break down the protective oxide film. When monitoring for corrosion, look for reddish brown iron oxide deposits and for pits which may have formed in a metal surface.
Similarly, crevice corrosion is initiated with the breakdown of the protective oxide film and continues with the formation of shallow pits. Rather than occurring in plain sight, crevice corrosion occurs in crevices, i.e., on a metal surface in contact with another surface.
Galvanic corrosion occurs when two dissimilar metals come into contact with each other in the presence of an electrolyte, an electrically conducting fluid such as seawater. Galvanic corrosion leads to pitting and material loss of the lesser of the two metals.
Stress Corrosion Cracking
In a marine environment, certain alloys are susceptible to chloride-ion induced stress corrosion cracking (SCC). The chloride ion interacts chemically with with the material at a location of localized tensile stress, e.g., the tip of a crack where tensile stresses are highest. This failure mode is dangerous because it can destroy a component at stress levels below the yield strength of an alloy.
Corrosion can, in many circumstances, be minimized by providing your workforce with basic knowledge. Here, we examine several key topics.
First, consider the choice of materials for tubing applications, from the tubing to tube supports and tube clamps. 316 stainless steel tubing works well in many installations as long as it is kept clean and temperatures are not excessively high. In hot climates, especially in locations where salt deposits readily form and cannot be washed off, and also in installations where rust from carbon steel structural beams and floors accumulate on stainless steel surfaces, corrosion of 316 stainless tubing is more readily observed.
For these situations, tubing from superaustenitic or superduplex stainless steel offers much better corrosion resistance. The higher yield and tensile strength of superduplex stainless steel also make it easier to build systems that must be rated to a high maximum allowed working pressure.
When installing tubing, the use of tube support strips should be avoided because the relatively large crevice contact area makes it easier for a corrosive crevice solution to form. The industry is now moving toward the use of tubing clamps. This design minimizes clamp-to-tubing contact, which also facilitates visual inspection of tubing.
Placement and Design
Careful system design practices are necessary in corrosion prevention for two reasons: minimizing locations where crevice corrosion can occur and minimizing the contact of non-compatible metals susceptible to galvanic corrosion. One way to minimize crevices in a tube system is to avoid placing tubing directly against walls or against each other. When crevice corrosion of 316 stainless steel tubing is observed, one can replace 316 stainless steel tubing with more corrosion-resistant tubing, which can be installed with cost-effective 316 stainless steel tube fittings. Swagelok offers several engineered combinations of 316 stainless steel tube fittings with tubing from different alloys.
Training and Corrosion Programs
Beyond these simple measures, a best-in-class approach involves in-depth training and the implementation of a regular and robust corrosion-monitoring program. Swagelok educates its customers about corrosion because classroom learning and hands-on experience can be invaluable for operators and technicians. And while it’s common for a company to employ a corrosion expert, it’s just as common that such knowledge is not distributed widely throughout the operation. Building a basic understanding of corrosion—what it looks like, where it occurs, and for what reasons—among those who work with tubing systems every day can prevent many issues. Start saving resources by taking a closer look at your fluid systems—especially where it’s hard to see.
The Simple Science of Corrosion
What exactly is corrosion? Most engineered metals used in oil and gas applications are obtained from ores or other natural compounds, which we expend extraordinary amounts of energy to transform to their finished state, e.g., stainless steel. Corrosion occurs when these metals lose that energy and go back to a more stable state, such as when they were first obtained from the earth.
Stainless steels commonly contain a sufficient amount of chromium to form a protective oxide layer, which greatly reduces the natural tendency for corrosion to occur. Corrosion does occur, however, when environmental conditions cause that layer to break down. The process looks like this:
When combined with water and oxygen—found in most natural environments—iron and steel form hydrated iron oxides (rust). Through the formation of rust, the metal atom loses one or more of its electrons and leaves the bulk metal behind. The resulting loss of material reduces the thickness of the carbon and low alloy steels, which are affected by general corrosion, making them prone to mechanical failure.