The Science and Prevention of Offshore Corrosion for Safety and Performance

Corrosion occurs through electrochemical reactions wherein a metal sheds electrons and then reacts chemically. Scientists categorize the process into two different types of half reactions (or half-cell reactions). The first is called oxidation, which is when a metal loses some of its electrons due to the environment. When those electrons are transported through the metal to a nearby environment, they react in what's called a reduction reaction. The combination of these is called a reduction oxidation – or redox – reaction. The key result here is that the oxidation states of the reactants are changing as the electrons move from one to the other.

From a practical standpoint, this reaction consumes the metal material, which is the corrosion we're all trying to prevent. It can take the form of thinning the metal, leading to loss of pressure containment, or it can take the form of pitting, cracking, or crevice corrosion, any or all of which can result in mechanical reduction in the performance of the material and the component.

In an offshore environment, we address corrosion risks first by considering the two types: externally influenced corrosion and internally influenced corrosion.

From the Outside In, and the Inside Out

Externally influenced corrosion is a product of the environment. In addition to factors like salt water and sea spray, there are also harsh elements and compounds which are part of the actual production process. In the continuous wet-and-dry cycles of an offshore platform, corrosive substances may be relatively dilute when surfaces are wet, but become increasingly more concentrated as the water evaporates, increasing the severity of the environment.

Other types of externally influenced corrosion include fretting corrosion, which occurs when two components rub against each other and wear over time; and cyclical loading on components. That cycling results in fatigue, which can be accelerated in a harsh environment, which we call corrosion-assisted fatigue.

Internally influenced corrosion is directly related to the hydrocarbons that offshore assets are extracting from wells. There's a very broad range of chemical compounds to be found within these substances, and it can vary by location and application. These compounds may contain carbon dioxide, for example, which can form carbonic acid when it reacts with water; or hydrogen sulfide, which can produce sulfuric acid.

Additionally, the deeper the well, the greater the temperatures and pressures a system is handling. It's a very aggressive environment, and the combination of heat and the stress of pressure containment in a complex system can result in environmentally-assisted cracking.

External Corrosion: Mitigation by Type

Looking further at externally influenced corrosion, we can categorize it into a few types and approaches to mitigating them.

Uniform corrosion (or general corrosion) is one common type of externally influenced corrosion, and as the name implies, it's a case where an agent or factor is consuming a material generally evenly wherever it's exposed. It's the easiest form of corrosion to spot and predict, and although it's unusual for general corrosion to lead to serious failures, the gradual decrease of component thickness needs to be considered when calculating pressure ratings. In a marine environment, unprotected carbon steel may recede by as much as 1mm ever year.

One mitigation route is simply using a thicker material so that the component's life cycle will be longer than the corrosion process. This preserves functionality over the life of the product. The same outcome can be achieved by selecting a more corrosion-resistant alloy during specification, or by painting the component with a resistant coating.

Galvanic corrosion is also common in offshore locations, and it occurs when particularly dissimilar metals are in contact in the presence of an electrolyte such as seawater. Metals can be rated on a scale called the Anodic Index, which provides a measure of susceptibility to galvanic corrosion. The further apart two metals are on this index (measured by voltage) the greater the chances of galvanic corrosion taking hold if they're in contact like a bolt holding a plate together.

Materials with greater than a 0.2V difference on the Anodic Index can actually accelerate the corrosion in the weaker material if they're in contact, which means mitigating galvanic corrosion is as straightforward as selecting the proper materials for coupling.

Pitting corrosion and crevice corrosion are both locally-influenced corrosions that begin with the breakdown of the passive oxide layer on a metal's surface. Pitting corrosion causes small cavities to form on the surface of a material. These pits – often observed in high-chloride environments at elevated temperatures – can grow deep enough to perforate tubing as the iron chloride solution concentration increases and becomes more acidic as the pit deepens. Crevice corrosion occurs at far lower temperatures, and takes place in crevices such as those between tubing and tube supports, between adjacent tubing runs, or beneath accumulated dirt and deposits on surfaces. The tighter the crevice, the greater the risk for corrosion to occur, since seawater cannot rapidly diffuse out of the crevice.

Avoiding both pitting and crevice corrosion is a matter of selecting steel with a higher chromium level, which results in a more robust passive oxide film. Comparing the Pitting Resistance Equivalent Number (PREN) of materials will also help guide decisions. The higher the PREN, the greater the metal's resistance to pitting corrosion. Increasing molybdenum and nitrogen content can also boost the resistance of that passive oxide layer.

If you have a component that's under stress and exposed to a chloride rich environment or the dissolved salt in seawater, the combination can attack the material and cause cracking. Components with a higher nickel content can help prevent this chloride stress corrosion. For the most severe situations, nickel-based alloys are effectively immune to chloride stress corrosion cracking. Duplex stainless steels are another option, since they combine a relatively low amount of nickel with an increased amount of chromium, resulting in a duplex microstructure of ferrite and austenite together in an alloy that offers excellent resistance.

It's important to note that the ASTM standard requirements for 316 stainless steel specify a nickel level range of 10-14% and a chromium level between 16-18%. Since these two ingredients are key to corrosion resistance and ductility, Swagelok specifies its material to a minimum of 17% chromium and 12% nickel, rather than the ASTM minimum threshold.

Fretting and fatigue, noted earlier, can also be mitigated early in the process through proper system design. Tubing runs should be engineered to avoid the physical contact which causes fretting, and to make sure tubes are supported at regular intervals to prevent excessive vibration. Reducing vibration also prevents accumulated fatigue damage that can happen when a tube run comes out of a compressor, for example, or due to the vibration or swaying of the offshore platform itself.

Material Matters: Specifications to Prevent Internal Corrosion

As noted, internally influenced corrosion occurs due to the nature of deep-well extraction and the fluid that's being drawn up from within the earth. It's a very complex environment, from the seawater and its chloride ions to the well composition and the risks of various acids forming. And if operators are mitigating corrosion with cathodic protection methods, that can sometimes result in hydrogen being present, adding another variable to the mix. To these several types of corrosion effectively acting together, add the factors of pressure containment and tensile load, and conditions are ideal for environmentally assisted cracking and accelerated corrosion.

Addressing these issues begins with referring to NACE MR0175 / ISO15156 standard, which describes the limits for material use in oil and sour gas well environments. These guidelines lay out the thresholds for the types of alloys to use as a function of temperature, pH or acidity, choride concentration, and hydrogen sulfide concentration. The standard puts limits on whether suitable materials have been heat treated in certain ways, and what strength they are. There are also limitations on whether materials are for general use or for downhole.

All of this means that having a strong understanding of your environment is absolutely critical, especially in the earliest stages of engineering, procurement, and construction of an offshore asset. The right fluid system experts can provide crucial guidance in navigating compliance and developing a selection of materials and heat treatment to prevent corrosion, ehancing safety and preserving uptime under some of the world's toughest conditions.