Signs and Symptoms

Corrosion is perhaps one of the more obvious signs of plant ageing because of visible signs of corrosion product, either external or internally within equipment. The nature of many materials, especially carbon and low alloy steels, is to react with the environment by a corrosion process to attain a more stable condition, e.g. metallic iron “wants” to become iron ore again.

Many equipment items take account of this in the design process e.g. corrosion allowance so it is important to note that the presence of corrosion products, i.e. rust, does not indicate that equipment is not fit for its service. Rust is merely a sign that the equipment is ageing. The rate of this ageing process and its importance in risk terms are parameters the plant operator should be concerned with.

Susceptibility 

All metallic materials are susceptible to corrosion and/or corrosion cracking. Materials termed

“corrosion resistant alloys” or CRAs are less susceptible but not immune. This class nof materials are protected by a corrosion process that forms a thin layer of metal oxide at the  surface. Should the layer be damaged in an environment that does not support re-oxidation, then the material can become susceptible to corrosive attack.

Management Options 

Corrosion can be prevented or monitored and controlled. Prevention methods include 

coatings and/or cathodic protection (often termed “CP”). CP can be achieved either by the 

use of impressed currents or by connection of sacrificial anodes typically made from zinc or 

aluminium blocks. If coatings are used there should be evidence of coatings inspection and 

if CP is employed evidence of maintenance and monitoring of CP effectiveness should be 

available. 

For monitoring and control, management of corrosion is achieved through the following 

processes: 

• Identification 

• Detection 

• Quantification 

• Assessment 

Identification usually involves a risk assessment, e.g. RBI plan or may take the form of asset 

registers arranged to identify those equipment items that are expected to corrode in one way 

or another. Detection is the application of a suitable inspection technique, often visual, that can locate the corrosion. 

Quantification is achieved by measuring the remaining thickness of material available to 

contribute to the overall structural integrity of the equipment. In some instances, engineering 

judgement is applied but this should be documented to a sufficient extent that reasonable 

next inspection intervals can be deduced. 

Source:http://www.hse.gov.uk/offshore/ageing/ageing-plant-summary-guide.pdf