Structural or equipment failure as a result of corrosion and/or fatigue can of course have a serious environmental impact; with any substantial leakage from a sub-sea oil well, transmission pipeline, storage vessel or other equipment likely to lead to pollution. However the nature and rate of corrosion fatigue varies depending on the environment and application. For example, salinity, temperature and oxygen levels are all factors for corrosion.
One of the most susceptible areas on a platform is the splash zone, where water and oxygen are in abundance, and where evaporating seawater concentrates salts. Another susceptible area is just below the sea surface, where oxygen-in-water levels are high and the sea temperature is relatively warm, compared to at depths.
However, a lack of oxygen does not negate the risk of corrosion. For example, there are many microbes existing at depths where oxygen levels are minimal. In the presence of these microbes the requirement for oxygen in the corrosion reaction is removed, and microbial induced corrosion (MIC) can be extremely damaging.
Corrosion can also be induced by connecting together dissimilar metals. Known as biometalic or galvanic corrosion, this phenomenon is actually exploited in the form of 'cathodic protection'. Here, sacrificial anodes, fabricated from materials such as zinc and aluminium, are attached to underwater structures made of iron-based metals. The galvanic couple formed by the introduction of these anodes onto the structure separates the electrochemical half-cell (redox) reactions such that the oxidation reaction, which is responsible for the damage, occurs on the anodes and the reduction reaction takes place on the structure being protected.
Areas of research
Understanding the nature of corrosion is one of the remits of the Centre for Corrosion Technology (CCT). It was founded in 1988 and is part of the Materials and Engineering Research Institute (MERI), which operates out of Sheffield Hallam University, in the UK. The centre has test facilities which are in regular use for monitoring and quantifying the corrosion degradation (mechanisms and rates) of materials and coatings, and has a long-standing relationship with the oil and gas industries. Indeed, CCT has collaborated with virtually every type of company involved in the oil and gas sector.
There are of course numerous materials used in offshore applications. These materials include carbon steels, a variety of stainless steels (such as marine-specific duplex and superduplex) and nickel-based alloys. Coatings are common-place too, and include paints and hot-dip and electroplated surfaces. Note, the CCT is active in developing coatings which are set to greatly benefit marine industries.
All of the above (materials and coatings) are susceptible, to a greater or lesser degree, to corrosion or corrosion-induced degradation - and recognised forms include pitting, crevice, galvanic and intergranular corrosion. In general, the issue is not so much why or how they corrode, but how quickly; and the CCT regularly performs accelerated environmental tests, including neutral salt spray and humidity testing. However, while accelerated exposure tests can be useful in ranking materials it is not possible to use such methods to predict the lifetime of a structure or component or to provide any information of the kinetics of any localised corrosion that may be occurring on the surface. Fortunately, the electrochemical nature of the corrosion process can be exploited in order to characterise the behaviour of a metal in a given environment. For example, direct current (dc) perturbation methods can be used to characterise metal surfaces in terms of their corrosion behaviour under a range of different conditions (which may be applied to the operating environments to be encountered by the structure or component). A frequently applied dc perturbation method is the the Linear Polarisation Resistance (LPR) technique, which provides a quantitative value for the corrosion rate of a metal in a given environment.
Electrochemical Impedance Spectroscopy (EIS) is also a useful, and powerful, technique. Used to characterise a coated system, it provides information regarding efficiency, integrity and breakdown of coatings. Further, localised corrosion measurement techniques are able to identify specific corrosion events on the surface of a sample. For example, a technique known as the scanning vibrating electrode technique (SVET) utilises the electrical field set up in a solution above a metal (corroding in the solution). SVET can be used to produce a time-resolved representation of the changes in corrosion activity.
As for the other enemy, fatigue, it comes about as a result of wave loading and, above the surface, winds. Wave loading in particular produces cyclic stresses that can lead to fatigue cracks; which in turn may be attacked by the highly corrosive environment.
Take sub-sea risers, for example, they experience a wide range of environments from the oxygen-rich, warm surface waters down to the microbe-rich silts of the sea bed. Risers are typically reinforced by steel cables, wound around the inner pipe, and are virtually always in motion. This motion causes a mechanism known as fretting; very small amplitude movements between surfaces that are in intimate contact.
Fretting, the study of which falls under a discipline known as tribology, removes the protective oxide layer from the surfaces of the cable which are in contact, exposing them to the corrosive environment. This exposure, plus heat from the oil (typically up to 120°C), can compromise the riser's integrity. Working with a oil producer, CCT has conducted validation exercises for different materials, from which future reinforcing cables are likely to be made.
Recently, researchers at MERI developed a series of low-temperature cure hybrid sol-gels, which contain both inorganic and organic components. Where marine applications are concerned, the greatest potential comes from the fact that the organic component is typically carbon-based, and can therefore include a 'living' element; namely bacteria. This allows for the production of 'biologically active' coatings which would prevent/limit the colonisation and build up of a deleterious biofilm. A number of field tests have been conducted to evaluate the properties and performance of hybrid sol-gel coatings - the most notable of which so far has been at the Thames Barrier in London. Above, the three plates shown were submerged for six months at the Thames Barrier. The results show clearly corrosion of the bare aluminium sample (B), some protection with the sol-gel coating without bacteria (C) but no corrosion/biofouling of the biocoat sample (A).
Dr David Greenfield and Dr Chris Sammon are with the Materials & Engineering Research Institute (MERI), Sheffield Hallam University, Shefiield, UK. www.shu.ac.uk/meri