Greater focus on external corrosion is needed
Inspection and maintenance to counter corrosion support safe, cost-effective operation in process industries. For internal corrosion, industry guidance and factors like fluid type, pressure and temperature help asset integrity specialists to decide which threats are credible.
It is rare to find a similarly detailed focus on analysing and predicting external corrosion, however. The internal form is seen as statistically more likely and is easier to quantify and predict. Yet external corrosion is a common threat to almost all equipment in process industries where the cost of a major incident caused by hydrocarbon releases can run into billions of dollars. It is worth asking if we are so used to seeing rust on familiar items like cars in our everyday lives that it can become effectively invisible to us, even on process equipment.
Getting to grips with external corrosion
Well-established risk-based inspection codes such as API 581 address general external corrosion attacks and corrosion under insulation (CUI). They involve considering factors such as climate, condensation and how close equipment is to sources of potentially corrosive water mists, such as cooling towers.
Even though general external corrosion can often be slow, it can do much damage over a plant’s lifetime. It can be aggressive, especially along coasts in hot climates when salt fogs wet steel surfaces in the morning. Water evaporation during the day drives up the salt concentration to cause high rates of corrosion that damages coatings.
High external concentrations of sulphur oxide pollution greatly accelerate external corrosion of plant by acidifying water and destabilizing rust layers. External corrosion can also be high inland. After a big storm in 1987 in the UK, a measurement in central London found that, due to salt transport from sea to city, the external corrosion rate was equivalent to that next to a beach.
Our failure analysis laboratory in Loughborough sees all of this clearly. It has investigated major issues with external corrosion of flange faces, bolting and piping for various industries. Most recently, our colleagues completed a joint industry project on CUI which, just like Corrosion Under Pipe Supports (CUPS; refer to image A), can rapidly lead to serious piping damage, potentially leading to pipe failure. CUI is thought to account for 40% to 60% of pipework maintenance costs.
Choosing coatings to protect against external corrosion
Process industries coat and paint vessels and pipework as a main defence against external corrosion. NACE International estimated that process industries in the US alone spent USD260 million in 2013 on coatings for this purpose.
This can work well if the coating is carefully selected and qualified for site conditions. In some failures, a coating against atmospheric corrosion was used on cool pipework exposed to heavy condensation from a humid atmosphere, while in other cases an immersion-grade coating is required.
Consistent approaches to assessing external corrosion
Coating systems need inspecting regularly. Many operators have internal standards for assessing the severity of corrosion and coating degradation such as the ISO 4628.
To assess degrees of coating degradation and substrate surface rusting, the standards use a comparison library of pictures showing coatings and steel substrates with varying levels of damage. However, they do not address the depth of attack, and the severity judgement depends on the experience of inspection personnel. It is important to ensure a consistent approach throughout the analysis, which is why our laboratory uses trained assessors to conduct site surveys for external corrosion, CUI and CUPs. Find out more: