Chrome steel elements could degrade over time when they’re uncovered to inside and exterior elements that trigger corrosion, which may price amenities vital money and time. Deteriorated components could require repairs or alternative, both of which can lead to unplanned downtime for the system concerned (Fig. 1).
Services ought to deploy elements of their fluid techniques produced from 316 stainless-steel, whose chromium ranges present additional safety towards the injury corrosion can inflict.
In accordance with the Nationwide Affiliation of Corrosion Engineers (NACE), corrosion can price offshore and nearshore amenities greater than $1 billion every year. Happily, a number of easy options exist that may mitigate or remove the issues earlier than they turn into insurmountable for oil and gasoline corporations.
Earlier than particular identifications are made, groups should perceive what causes corrosion and what steps they will take to stop it from worsening and placing the fluid system vulnerable to failure. Then upkeep groups ought to determine which form of corrosion is harming their fluid techniques by studying and understanding what units the differing types aside. As soon as a whole understanding of which corrosion is affecting a system is reached, then groups can take motion.
Why stainless-steel corrodes. Virtually each metallic on the planet can corrode beneath sure circumstances, however the harsh circumstances of oil and gasoline purposes current particular challenges particularly for offshore installations.
Corrosion outcomes from electrochemical reactions and oxidation (lack of electrons) at an anode, or discount (gaining of electrons) at a cathode (Fig. 2). A standard instance is iron tubing, which can oxidize and launch two electrons. Water launched to the system could cause the iron to dissolve into Fe2 optimistic ions. On the identical time, the electrons could trigger a discount response, altering dissolved O2 into OH-, a negatively charged hydroxide ion.
Each pitting and crevice corrosion start when the oxide layer on the skin of the metallic floor breaks down and creates area for corrosive supplies to do injury.
Prone metallic tubing will be present in analytical and course of instrumentation, hydraulic traces, and management and utility purposes within the oil and gasoline industries. To keep away from preliminary corrosion injury, most techniques are designed with stainless-steel that has a minimal of 10% chromium in its composition. Ample chromium ranges induce the creation of an oxide layer that helps sluggish and even stop corrosion. Even the strongest stainless steels can succumb to corrosion if environmental circumstances destroy that oxide layer. With out that layer, corrosion reactions could proliferate shortly.
The 2 most typical corrosion sorts dealing with the oil and gasoline trade are pitting corrosion and crevice corrosion.
The variations between pitting and crevice corrosions. At many oil and gasoline amenities, a number of corrosion sorts could occur concurrently and destroy total fluid techniques within the course of due to the operation surroundings, supplies used, and course of fluids. The 2 most typical types of stainless-steel corrosion are pitting corrosion and crevice corrosion (Fig. 3).
Pitting corrosion: Pitting happens when the chromium layer is destroyed over time, leaving the naked metallic beneath unprotected. When the naked metallic is uncovered to corrosive options, injury will happen. Usually, the injury takes the type of small cavities, generally often known as pits.
Pitting corrosion (left) and crevice corrosion (proper) symbolize vital dangers to offshore oil and gasoline fluid techniques. Services ought to make sure that their groups are outfitted with the information to determine, restore, and forestall both sort of corrosion.
Visible inspection could reveal the beginnings of pitting corrosion, however the quantity of misplaced materials beneath the floor can silently undermine a pipe’s efficiency. Within the worst-case eventualities, pitting corrosion can perforate tube partitions and end in pricey leaks. Left unaddressed, pitting corrosion can result in cracks in elements beneath tensile masses. If the surroundings comprises excessive ranges of chloride (CI-), comparable to seawater hitting offshore drilling platforms, pitting corrosion is much more doubtless, particularly if excessive temperatures are concerned.
A transparent sign that pitting corrosion is happening is reddish-brown iron oxide deposits on the floor in addition to the beginnings of precise pits. If CI-bearing water like seawater swimming pools and evaporates, the remaining resolution will turn into much more corrosive. Hold upward-facing surfaces away from standing water to stop CI-induced pitting corrosion.
Crevice corrosion: The reason for crevice corrosion is much like pitting corrosion, which means that the oxide layer has damaged down. What makes crevice corrosion extra insidious is that it hardly ever occurs in plain sight. As a substitute, because the identify implies, it happens in crevices, making it more difficult to search out and forestall. Moreover, the large and comparatively shallow pits that happen in crevice corrosion solely develop as soon as the method has begun. In most fluid techniques, crevices are created between tubing and tube helps or clamps, between adjoining tubing runs, and beneath dust and deposits which have collected on the floor.
Irrespective of how intelligent the design, crevices will inevitably occur. The tightest crevices pose the one biggest hazard to the integrity of the system’s stainless-steel elements. Crevices are significantly problematic in offshore purposes as a result of seawater can diffuse into the crevice with out entry to an outlet. The ensuing chemically aggressive surroundings doesn’t permit corrosion-causing ions to come back again out, leaving the complete floor prone to fast corrosion.
Moreover, crevice corrosion typically stays hidden till a tubing clamp is eliminated, leaving it probably undetected for lengthy durations of time. Not like pitting corrosion, crevice corrosion happens at decrease temperatures as a result of it’s simpler to create a pit beneath a tube clamp or different such machine.
Preserving corrosion from occurring. The best strategy to decrease corrosion is to coach upkeep groups about primary supplies information and instituting corrosion-prevention requirements for the power.
First, contemplate the selection of supplies for tubing purposes, from the tubing itself to tube helps and clamps. Laboratory testing for essential pitting temperature (CPT) and important crevice temperature (CCT)—based on the ASTM G48 commonplace—is a useful instrument for evaluating supplies for use in corrosive environments. CPT testing evaluates the temperature at which pitting begins on a cloth in a selected corrosive resolution. Equally, CCT testing evaluates at what temperature crevice corrosion begins when a predefined crevice is positioned on a metallic pattern in a corrosive resolution.
Supplies which have excessive values for CPT and CCT are typically extra appropriate to be used in related corrosive environments than supplies with low values. For instance, 304L has the bottom CPT worth of the supplies proven in (Fig. 4) whereas 6Mo and 2507 have the 2 highest CPT and CCT values. This means that 6Mo and 2507 are more likely to be extra immune to pitting and crevice corrosion than 304L and 316L in chloride-bearing options. It is very important needless to say these assessments are helpful for comparability and materials choice however aren’t predictive of when a cloth will fail in a real-life software.
Essential Pitting Temperatures (CPT) and Essential Crevice Temperature (CCT) are necessary values used to find out which supplies will finest resist corrosion in harsh working environments.
The 316L grade of stainless-steel (UNS S31603) tubing works nicely in lots of installations so long as it’s stored clear, and temperatures aren’t excessively excessive. In hotter climates, particularly in areas the place salt deposits readily type and in installations the place rust from carbon metal structural beams and flooring accumulates on stainless-steel surfaces, corrosion of 316L stainless-steel tubing is extra readily noticed. Nonetheless, as a result of useful addition of molybdenum, 316L sometimes performs higher than 304L (UNS S30403) stainless-steel in these corrosive environments.
For conditions the place 316L is inadequate to fulfill the lifetime necessities of the applying, tubing produced from tremendous austenitic (e.g., 6Mo or 6HN, UNS N08367) or tremendous duplex (e.g., 2507, UNS S32750) stainless steels affords considerably improved corrosion resistance. Moreover, the upper yield and tensile power of tremendous austenitic and tremendous duplex stainless steels make it simpler to construct techniques that should be rated to a better most allowable working stress (MAWP). Working together with your tubing and tube fittings provider for steerage in deciding on the fitting merchandise and supplies will help you keep away from pricey errors.
Along with supplies choice, cautious system practices are obligatory for stopping corrosion and minimizing the variety of areas the place crevice corrosion can happen. One strategy to mitigate crevice corrosion in a tube system is to keep away from putting tubing instantly towards partitions or towards one another. When crevice corrosion of 316L stainless-steel tubing is noticed, one can exchange 316L tubing with extra corrosion-resistant tubing comparable to 6Mo, which will be put in with cost-effective 316L tube fittings in advised mixed-material engineered mixtures.
Why it issues. Oil and gasoline corporations are more and more beneath stress to maintain upkeep prices down, and sustaining fluid techniques at peak efficiency is a necessary manner to assist accomplish it. In actuality, meaning ensuring the upkeep crew understands the right way to determine, right, and forestall corrosion earlier than it damages a complete system.
Constructing a primary understanding of corrosion—what it seems like, the place it happens, and for what causes—amongst those that frequently work with tubing techniques will help stop materials failure and expensive repairs in addition to enhance system longevity.
