Identifying problems early empowers technicians to save facilities time and money while improving safety for everyone involved.
By Buddy Damm, Senior Scientist, Swagelok Company
Metal components in industrial fluid systems often have to operate in harsh conditions. If left unchecked, adverse conditions can trigger and aggravate the corrosion of these components and cause physical and financial harm to an operation. In metal tubing, for example, the National Association of Corrosion Engineers (NACE) estimates the profit loss to offshore and nearshore operations from corrosion is more than $1 billion per year.
Recognizing corrosion early and addressing the root causes can help mitigate the worst damage, before repair and replacement costs accumulate. It is imperative that facilities ensure their technicians can identify the most common types of corrosion quickly, and understand what actions to take. Technicians can often take simple actions to remediate the damage if they know what to look for.
Two common types of corrosion are pitting and crevice, and they are normally responsible for the most expensive damage over time. Before determining how to fix the problem, it is important to understand why corrosion occurs, how pitting and crevice corrosion differ, and how to protect components from corrosion, to help avoid premature failures and costly replacements.
Why Stainless Steel Corrodes
All metals are liable to corrode when exposed to certain operating conditions, but proper planning and preventive maintenance can keep the damage to a minimum. Taking simple steps to halt corrosion before it spreads requires that technicians possess knowledge of how different types of corrosion occur, and which surfaces should be most frequently examined. Careful observation and remediation efforts can save time and money in the long run.
Corrosion is a set of electrochemical reactions that involve oxidation (loss of electrons) at an anode and a reduction (gaining of electrons) at a cathode (Figure 1). For example, iron in tubing may oxidize, yielding two electrons and dissolving into water as a Fe2+ (positive) ion. Simultaneously, the electrons from oxidizing iron may participate in a reduction reaction that uses O2 dissolved in H2O to form OH- (negative) ions.
The most common applications for metal tubing are analytical and process instrumentation, hydraulic lines, and control and utility. These tubes are often made from stainless steel with more than 10% chromium (Cr), which builds a passive oxide layer on the surface and inhibits corrosion (Figure 2). Stainless steel corrosion does occur, however, when environmental conditions or mechanical damage causes that layer to break down. If the protective oxide cannot reform in a given solution, corrosion reactions may progress rapidly. In both pitting and crevice corrosion, the local breakdown of the passive oxide layer creates a region where corrosion damage is greatly accelerated.
How Pitting Corrosion and Crevice Corrosion Differ
The types of corrosion possible on oil and gas installations are myriad and depend on what component materials are used, the operating environment, and what fluids the components transport. However, two forms of corrosion are most common: pitting corrosion and crevice corrosion.
Pitting Corrosion – This type of corrosion occurs when the protective chromium-rich oxide layer on the surface of stainless steel breaks down, allowing the bare metal underneath to become susceptible to continued attack in a corrosive solution (Figure 3). Cavities form as the stainless steel corrodes and creates pits.
Although the entry point of a pit may be detectable via thorough visual inspection, there may be a deep network of lost material lurking below the surface. Without appropriate remediation, pits continue to expand and can sometimes create holes in tubing walls. If process fluids leak out of these holes, it costs money in lost fluid, creates environmental and safety hazards, and can be expensive to fix. Additionally, pitting corrosion can cause cracks in already strained components. Environments with higher chloride (Cl-) concentrations, including those created by evaporation from deposited saltwater droplets, are likely to cause pitting corrosion – especially at high temperatures.
The most obvious sign of pitting corrosion is reddish-brown iron oxide deposits and pits on the metal surface. Special attention ought to be paid to upward-facing surfaces where chlorine-bearing water (e.g., seawater) may pool and evaporate, or downward-facing regions where hanging droplets dry. As water evaporates in these regions, the chlorine concentrations in the remaining water will increase and become more corrosive, resulting in pitting corrosion.
Crevice Corrosion – This type of corrosion is similar to pitting corrosion; the protective oxide film on components deteriorates as it is exposed to corrosive fluids. The particularly insidious nature of crevice corrosion is that it frequently occurs out of sight, in crevices under clamps or similar tight spaces, making it hard to identify (Figure 4). After crevice corrosion has started, wide and shallow pits form, weakening the component material and increasing the chances of mechanical failure.
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. No matter how well the system is designed, crevices are practically inevitable, and the tighter they are, the harder they are to service. Catching crevice corrosion is far trickier than seeing pitting corrosion on the surface and, because it happens out of sight, it can quickly devastate systems.
In coastal or offshore applications, crevice corrosion often occurs when seawater diffuses into a crevice, leading to a chemically aggressive environment out of which corrosion-causing ions cannot readily diffuse. As crevice corrosion progresses, the solution within the crevice becomes more corrosive. In such a scenario, the entire surface within the crevice can corrode at an ever-increasing rate.
Typically, crevice corrosion is discovered when technicians remove clamps from existing tubing runs. It usually occurs at lower temperatures than pitting corrosion because it takes less energy for corrosion to occur beneath geometric crevices like those created by the tube clamp.
Keeping Pitting and Crevice Corrosion at Bay
Education is key to keeping corrosion damage to a minimum. Arming system designers and technicians with appropriate materials science knowledge will advance those goals, as will having strict corrosion-prevention protocols in place.
As systems are being designed, choosing the right materials for tubing and tube supports is crucial. The first line of defense is to consider the Pitting Resistance Equivalent Number, or PREN, which is an algebraic function of the Cr, Molybdenum (Mo), and Nitrogen (N) content of stainless steel. A higher Cr content results in a more Cr-rich passive oxide, and Mo and N enhance the robustness of the passive oxide film. When pitting or crevice corrosion is observed, replacing components made from an alloy with a higher PREN will enhance pitting or crevice corrosion resistance.
The ASTM G48 standard for laboratory metrics of critical pitting temperature (CPT) and critical crevice temperature (CCT) is an invaluable tool for comparing materials for use in corrosive environments. CPT testing evaluates the temperature at which pitting begins on a material in a specific corrosive solution. Similarly, CCT testing evaluates the temperature at which crevice corrosion begins when a predefined crevice is placed on a metal sample in a corrosive solution. Because the local environment in a crevice becomes more aggressive than in a pit, the CCT is always lower than the CPT.
The higher the temperatures for CPT and CCT, the better those metals are for use in corrosive environments like offshore oil and gas platforms. For example, 304L has the lowest CPT value of the materials shown in Figure 5, while 6Mo and 2507 are the two highest for CPT and CCT temperatures. This indicates that 6Mo and 2507 will be more resistant to pitting and crevice corrosion than 304L and 316L in chloride-bearing solutions. Keep in mind that CPT and CCT values are comparative and not predictive so, even armed with this understanding, further investigation should occur before choosing components for specific applications.
316L austenitic stainless steel (UNS S31603) tubing and fittings are effective in resisting pitting and crevice corrosion in many service environments. If the oil and gas platform is in a warmer climate and is prone to saltwater pooling and drying, pitting or crevice corrosion of 316L stainless tubing is more readily observed. However, due to the beneficial addition of Mo, 316L typically performs better than 304L (UNS S30403) stainless steel in these corrosive environments. Furthermore, 316L with a minimum of 17 weight percent Cr can help to enhance localized corrosion resistance. The ASTM standard minimum is 16 weight percent.
For situations in which 316L cannot sustain its corrosion resistance over the tubing’s expected life cycle, tubing made from super austenitic (e.g., 6Mo or 6HN, UNS N08367) or super duplex (e.g., 2507, UNS S32750) stainless steels may be a better choice, as these alloys have higher Cr and Mo contents, and the addition of N. Additionally, the higher yield and tensile strength of super austenitic and super duplex stainless steels make it easier to build systems that must be rated to a higher maximum allowable working pressure (MAWP). Working with a reliable tubing and fitting supplier who can guide teams through appropriate component choices will keep facilities from having to engage in expensive retrofits.
While choosing the right initial materials can alleviate some corrosion concerns, facilities must also implement precise protocols to prevent corrosion and keep crevices to a minimum. One way is to avoid putting tubes directly against walls or each other. If crevice corrosion occurs in 316L stainless steel tubing, teams should consider replacing it with 6Mo tubing, which is more corrosion resistant. If properly engineered from the beginning, 316L fittings can be used despite the differences in metals.
Finding a supplier who can offer training in understanding corrosion (Figure 6) — from what it looks like to where it occurs and why — is essential to building corrosion-resistant systems from the ground up. Ensuring designers and technicians who manage the system have this basic knowledge can prevent material failures and keep systems operating at peak performance for longer. Proactive measures can prevent expensive repairs and minimize downtime. Keeping corrosion from spreading will improve profitability, keep employees safer, and improve overall system performance.
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