The billion-dollar question - high-integrity alloys for corrosion protection
Choosing the right high-integrity alloys for corrosion protection is a billion dollar decision. Alan Robinson, Managing Director at Arc Energy Resources, examines the options.
Both corrosion and its prevention cost industry billions of dollars every year. As such, careful decision making is vital when selecting materials and preventative processes used to extend the operating life of materials. A meticulous approach will ensure both cost-effective manufacture and safe long-term operation of equipment such as pipelines and valves, especially in deep-sea operations.
Consider that about two thirds of world nickel production goes into stainless steels. These are commonly used to produce corrosion-resistant components and equipment for industries such as oil, gas and nuclear, using the weld overlay cladding process. Duplex steels and nickel-based alloys are the only materials in general production that, when welded, can match the strength of carbon steels. This is especially relevant where weld overlay cladding can be specifically applied to areas under attack, and pertains to large quantities of fittings, or large forgings or castings.
Where maximum protection is needed, corrosion resistant alloys (CRAs) such as austenitic (300 series) stainless steels, ferritic/martensitic (400 series) stainless steels, duplex stainless steels and the more complex high-nickel chromium alloys must be considered. All are affected by the price of nickel, but their application as cladding materials does reduce the impact of rising costs, while providing the assurance of a heavy duty, metallurgically bonded protective layer that will not be undermined or dislodged in service.
This is certainly true when weld overlay cladding is used to protect oilfield valves from corrosion. As oil and gas wells are sunk deeper, and produce hotter and more aggressive cocktails of corrosive media, equipment manufacturers are forced to specify highly alloyed materials for valves and associated components to overcome the inevitable corrosion problems. The alloys used to manufacture the valves and associated components include carbon and low-alloy steels, and stainless steels. All can be overlay-clad to provide additional corrosion resistance in specified areas – or indeed over all surfaces in the case of the carbon and low-alloy varieties – to ensure total resistance.
Even when repairing equipment, affected areas can often be pre-machined and, using automated weld overlay cladding or specialised manual welding, rebuilt with a CRA – for example complex nickel aluminium bronze. Furthermore, the repair will usually be superior to the original metal.
Assessing the options
Consider the problems in the oil and gas industry. Hydrogen sulphide (H2S), dissolved CO2 and various chlorides are all present in the fossil fuels being delivered from subsea fields, as are high pressures and high temperatures. A sour service at high temperature is more corrosive, while the same service at high pressure is more erosive. A combination of the two is potentially expensive and hazardous, and will have cost implications on choices such as protecting low-cost carbon steels or manufacturing in high-cost CRAs.
When assessing corrosion protection for any production system, process engineers have a number of options. The effectiveness of each will vary depending on factors including:
- the aggressiveness of the product
- pressure and temperature
- the size and complexity of the system
- projected life expectancy
- the development period available
- overall budget constraints
So, how can welding engineers help industry to resist these attacks? Protection, where risk of attack is low and lifecycle is relatively short, may be as simple as an injected inhibitor used with conventional high-strength carbon or low-alloy steel. Where greater protection is needed, CRAs must be considered.
Duplex steels and nickel-based alloys (such as alloy 625) are the only materials in general production that, when welded, will achieve suitable levels of protection. However, there are constraints on the use of these materials in their solid form – namely cost, availability and the need for very tightly controlled welding procedures. Cost is particularly relevant where large quantities of components are needed, or when large forgings or castings are used. Wellhead valve systems and pipe bundle bulkheads are typical examples of this.
The use of carbon and low-alloy steels clad with a CRA has been common practice for some years, and is now a proven economical and technical alternative to solid alloys. It can offer the benefits of strength and availability of base materials combined with corrosion resistance applied in selected areas.
Weld overlay cladding technology presents the materials engineer with a wide choice of processes and great flexibility. An almost infinite range of component shapes and sizes can be protected, with an equally wide range of base material and cladding alloy alternatives. Selection of the most appropriate welding process is largely dependent on several factors, including:
- the size and geometry of the clad area
- access to the area to be clad
- alloy type
- specified clad thickness
- chemical composition limits
- welding position
- NDT acceptance standards
Internationally there are many welding processes in common use, but given that the process used must be practical, viable and provide the mechanical and chemical conditions to achieve service requirements, economics dictate that the higher deposition rate processes should prevail.
The gas tungsten arc welding (GTAW) processes provide excellent control and produce a high-quality product. Suitable for use in bores as small as 20mm, they are ideal for components of varied geometry where the position of the welding head must be frequently adjusted.
Plasma-transferred arc welding provides an equally attractive, quality product. Equipment costs are higher and the process variables are slightly more complex than GTAW, but the increased control offered by the arc makes it more amenable to computer numerical control (CNC).
For more open-access applications, the electroslag process offers an economically attractive solution. While not as fast, submerged arc welding using a solid wire consumable is a useful halfway house between strip cladding, and the slower GTAW and pulsed gas metal arc welding (GMAW).
When cladding high-strength (and, therefore, more hardenable) low-alloy steels such as AISI 4130, 21/4 Cr 1 Mo and, potentially, martensitic stainless steels such as A182 F6NM or AISI 410, post-weld heat treatment (PWHT) is invariably adopted. This stress relief ensures the layer of base material immediately below the weld (the heat-affected zone) is within the recommended hardness levels for the service conditions.
With so many options available and cost implications high, selecting materials and methods for corrosion prevention is a billion-dollar decision. But there is no doubting the ability of weld overlay cladding to protect off-the-shelf components of any shape and size with a wide variety of corrosion-resistant alloys, making it the most adaptable and flexible method in use.