When should you use corrosion resistant alloys in upstream production? CM
This is a wonderfully simple question that does not have a simple answer. Corrosion Resistant Alloys can be an excellent material choice for combatting corrosion and they are an important addition to the options that we have as Materials and Corrosion (M&C) professionals. However, as with all materials there are pros and cons and it is the role of M&C engineers to evaluate the best option.
It is important that we recognise that when we talk about a Corrosion Resistant Alloy (CRA) we are referring to a specific metal or alloy that is resistant to a specific corrosion mechanism or mechanisms. For example, in Upstream operations, for some applications,316L stainless steel is increasingly used for its resistance to carbon dioxide (CO2) corrosion, and can be considered a CRA for this threat. However, this alloy is vulnerable to corrosion in oxygenated sea water and so is not a good material for this environment. So before selecting a corrosion resistant material, in additional to many other parameters, it is very important we define the corrosion mechanism or mechanisms that it needs to resist – on both the internal and external surfaces of the equipment.As in the example above many CRA’s are stainless steels, and historically the term “stainless steel” was used generically to indicate corrosion resistant metals. The term CRA emerged when alloys that do not have iron (Fe) as their majority element started to be used, e.g. nickel and titanium based alloys.
For our colleagues who are not M&C engineers, the term CRA can be misleading because there is no metal or alloy that is immune to every corrosion environment. To make this point I have sometimes joked that CRA should mean “Can Rust Actually”. A key role of M&C engineers is to educate colleagues on the limits of CRA’s in particular environments.
A further consideration when selecting a CRA is that we do not eliminate one corrosion mechanism only for it to be replaced by another. For example many stainless steels are resistant to CO2 corrosion but vulnerable to cracking mechanisms (e.g. Chloride Stress Corrosion Cracking (SCC), Sulphide Stress Cracking (SSC)). In some cases it may be preferable to have a corrosion mechanism where the metal loss can be safely monitored with inspection versus a cracking mechanism which can occur suddenly and without any warning.
Other considerations when considering CRA’s are:
Cost: CRA’s are more expensive than carbon steels and so this needs to be factored into the economics of a project. This is known as “whole life costing” and includes procurement, construction, operation, and decommissioning costs. Typically, CRA’s become more attractive as the required lifetime of a project increases.
Welding: CRA’s require extra care when welding compared to carbon steels which adds to the cost of construction. It also requires welders who are more highly qualified and so another consideration is, will these welders and quality control personnel be available locally at the project site. This is often related to where in the world the project is located.
Galvanic Corrosion: Since CRA’s are more resistant to corrosion than carbon steel, problems can occur when they are joined to carbon steel or other more corrosive materials. The CRA can act as a cathode which promotes corrosion of the other metal. Incorrectly specified weld material is especially vulnerable to this, resulting in Preferential Weld Corrosion.
Construction & Commissioning: If a CRA is selected to resist a specific operational corrosion threat, such as CO2 corrosion, it is important to ensure that during the construction and commissioning phase it does not come into contact with a fluid that is corrosive to it. There are many examples where this has occurred. Probably the most well-known examples are where a stainless steel has been chosen for a pipeline for its CO2 resistance but during commissioning it has been hydrotested with poor quality water which contains chlorides and oxygen (and often bacteria) causing the stainless steel to corrode and/or crack before it ever enters operational service.
Availability: Due to manufacturing challenges and mill constraints, CRAs may not be readily available, and often have a long lead time.
Having considered all these factors let’s now return to the original question of when to use CRA’s in upstream production. Before considering CRA’s, it is usual to evaluate carbon steel as a base case for construction. Once this has been done CRA’s can be evaluated and compared to the carbon steel base case. Key considerations on top of design factors including mechanical properties, are:
Risk: CRA’s are often selected when the risk of premature failure of equipment made from carbon steel is unacceptable. If a project is expected to last 25 years but the mitigated corrosion rate of carbon steel will render the equipment unusable before that, then a CRA is likely to be a good option. Some engineers would like to be able to use a maximum, uninhibited corrosion rate for carbon steel as the trigger for moving to CRA’s, e.g. 3 mm/y. Whilst this provides a simple approach it is not the corrosion rate that is key – it is the time to failure that is important. Note that failure is not the point at which the equipment perforates and leaks ( often known as a loss of primary containment – LOPC), but the point at which it no longer meets the pressure requirements of the equipment. There are well known operations that have uninhibited corrosion rates of >5mm/y that are constructed from carbon steel in which corrosion is successfully mitigated to 0.1 mm/y
Operability: This is related to the risk section above but is worth noting separately. In many cases upstream equipment can be constructed from carbon steel and both internal and external corrosion successfully controlled using a combination of chemical inhibition, coatings, and/or cathodic protection. However, these mitigation systems are known as “active” mitigation and require monitoring, maintenance and repairs to be undertaken during the life of the operation. These require appropriately qualified personnel which incur operational costs. Moreover in remote land-based locations the facilities can be spread over very large areas, requiring staff and equipment to be driven for many hours – often called “windshield time” which is unproductive time and has associated safety risks. In such cases it may be more appropriate to select a CRA which would significantly reduce many of these activities and reduce the total risk of an operation. Similar arguments apply to subsea operations which are difficult and costly to access. Today, many subsea facilities are built entirely out of CRA’s for this reason.
Cost: This was discussed previously and is a key factor in deciding between carbon steel and CRA materials. The M&C engineer will need to look at the whole life costing of both options, which will then be assessed by the project commercial team, who will make a decision based on all project risks
In summary, hopefully it is clear that there are many factors to consider when selecting a CRA for upstream equipment. Sometimes the decision to use a CRA can be simple and examples of this include:
1. When corrosion rates of carbon steel are unacceptably high.
2. When the construction and operational costs for managing a carbon steel facility exceed the cost of using CRA for the project.
3. When the risk of maintaining an active mitigation system for carbon steel equipment is unacceptable.
4. And finally, when a combination of very high strength and corrosion resistance are essential.
However, it is far more common that the decision is not simple and it is the role of the M&C engineer to consider the risks and costs of both carbon steel and CRA options for a project. It is also usual that a single engineer will not have all the skills necessary to make this decision and will call on colleagues with specialised knowledge of materials and corrosion to support the evaluation.
There are many excellent sources for further information on CRA’s – some good starting points are:
Bijan Kermani, Fellows Corner, Corrosion Management, Issue 165, Institute of Corrosion Magazine, January/February 2022, page 19.
B. Kermani and D. Harrop, “Corrosion and Materials in Hydrocarbon Production”, published by John Wiley & Sons td, 2019. (note this book is an excellent reference and covers all aspects of upstream corrosion and materials topics).
Bill Hedges, Past President ICorr