Corrosion Resistant Alloys
Corrosion Resistant Alloys (also known as CRAs) provide essential long-term corrosion resistance to various components which are exposed to environments associated with oil and gas production. These are components such as downhole tubing and safety-critical elements, pipelines, heat exchangers, vessels, valves, wellhead components and valves, along with other facility equipment.
There are many different Corrosion Resistant Alloys to choose from, and they are usually categorised according to their resistance levels when exposed to particular environments.
Some key environmental parameters which play a part in influencing the corrosion properties of Corrosion Resistant Alloys include:
- Temperature
- Partial pressure CO2
- Whether sulphur is present or absent
- Environment pH
- Chloride ion concentration
- Partial pressure H2S
The above parameters can influence:
- The passive film’s stability (initiation of general corrosion or pitting)
- Repassivation ease of initiated pits
- Dissolution rates or metal from pits
- The Stress Corrosion Cracking (SCC) risk, initiating and propagating
CORROSION RESISTANT ALLOYS SELECTION METHODS
The methods of selecting Corrosion Resistant Alloys (CRAs) in order to produce and transport corrosive gas and oil can prove to be a tricky and complex problem solving task. If improperly executed, it can result in application errors and unreliable results about a CRA’s performance in a particular service environment.
Companies and individuals select CRAs in a number of different ways to cope with anticipated flowline and well conditions. If large research facilities are accessible, it is common procedure to develop and utilise a testing program to simulate conditions of a particular field environment (for example, flowlines vs. downhole). Based on the results, a group of alloys is then selected as a range of possible alternatives. Instead of testing all the alloys at once, it’s easier, simpler and more cost-effective to only test a few of the most likely candidate CRAs at a time.
This selection method could easily take one to three years to accomplish satisfactory results, and can equate to a considerable cost. Another way to select CRAs is the study of corrosion data applicable to the expected field conditions, which can easily eliminate any unsuitable candidates. Testing can then be carried out to refine the selection further.
Here are a few examples of Corrosion Resistant Alloys, and the conditions they are best applied to:
316L (Austenitic Stainless Steel)
Alloy 316L is commonly used for vessel cladding, surface piping and clad linepipe. As these are its main uses, care must be taken when ensuring the application is completely de-aerated. 316L will pit in the presence of oxygen, even when exposed to cold seawater.
Alloy 22
Alloy 22 displays exceptional resistance to a broad range of corrosive environments. It has excellent resistance to wet chlorine and mixtures containing nitric acid or oxidising acids with chlorine ions. Resistance to reducing acids such as sulphuric and hydrochloric can also be expected. Other corrosive chemicals to which the alloy has resistance are oxidising acid chlorides, wet chlorine, formic and acetic acids, ferric and cupric chlorides, sea water, brine and many mixed or contaminated chemical solutions, both organic and inorganic.
ZERON® 100
ZERON® 100 exhibits superior pitting and crevice corrosion resistance in warm seawater and has excellent resistance to stress corrosion cracking.
