Corrosion / selection of materials

This Technical Measures Document covers the corrosion of materials and the selection of materials of construction. Reference is made to relevant codes of practice and standards.

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The relevant Level 2 Criteria are:

Corrosion is the largest single cause of plant and equipment breakdown in the process industries. For most applications it is possible to select materials of construction that are completely resistant to attack by the process fluids, but the cost of such an approach is often prohibitive. In practice it is usual to select materials that corrode slowly at a known rate and to make an allowance for this in specifying the material thickness. However, a significant proportion of corrosion failures occur due to some form of localised corrosion, which results in failure in a much shorter time than would be expected from uniform wastage. Additionally, it is important to take into account that external atmospheric corrosion leads to many instances of loss of containment and tends to be a greater problem than internal corrosion. All these aspects of corrosive behaviour need to be addressed both at plant design time and during the life of the plant.

The operator should demonstrate that procedures are in place to ensure that corrosion and the selection of the correct materials of construction are considered at the process design stage. Additionally the operator should demonstrate that it has appropriate inspection and maintenance programmes in place in order to prevent corrosion causing loss of containment from its process operations. In doing so the following should be considered:

Corrosion in metallic components occurs when pure metals and their alloys form stable compounds with the process fluid by chemical reaction or electrochemical processes resulting in surface wastage. Appreciable corrosion can be permitted for tanks and piping if anticipated and allowed for in design thickness, but essentially no corrosion can be permitted in fine mesh wire screens, orifice plates and other items in which small changes in dimensions are critical. Rates of corrosion can be heavily affected by temperature changes and whilst a material of construction may be suitable at one temperature it may not be appropriate for use at a higher temperature with the same process fluid.

The corrosion of non-metallic materials is essentially a physiochemical process that manifests itself as swelling, cracking or softening of the material of construction. In many instances non-metallic materials will prove to be attractive from an economic and performance view.

The use of various substances as additives to process streams to inhibit corrosion has found widespread use and is generally most economically attractive in recirculation systems, however it has also been found to be attractive in some once through systems such as those encountered in the petroleum industry. Typical inhibitors used to prevent corrosion of iron or steel in aqueous solutions are chromates, phosphates, and silicates. In acid solutions organic sulphides and amides are effective.

There are many forms of localised corrosion than can lead to early failure of equipment. The prevention of corrosion should be addressed at the mechanical design stage and proper design to minimise local corrosion should include free and complete drainage, minimising crevices, no dead spots in pipework and ease of cleaning and inspection. Some of the more common types of local corrosion are briefly discussed in this section.

Pitting often occurs where certain impurities such as chlorides are present in process streams and cooling waters. This is an extreme form of localised corrosion. Once initiated pits are usually self-accelerating and can result in rapid failures.

Many metals suffer from stress corrosion cracking under certain conditions. In piping the most frequent failures from stress corrosion cracking occur with austenitic stainless steels in contact with solutions containing chloride. Even trace quantities of chlorides can cause problems at temperatures above 60°C.

Crevice corrosion may occur where liquid is trapped between close fitting metal surfaces, or between a metal surface and a non-metallic material such as a gasket. Attention to detail at the design and fabrication stage should be given to areas such as jointing to prevent crevice corrosion.

Localised erosion can occur where equipment orientation causes fluid velocities to accelerate such as at bends. Some chemicals can be handled in carbon steel piping because they form protective coatings of ferric compounds in pipework. Careful design to ensure the coating is not eroded is necessary.

Exterior surface corrosion or rusting of pipework occurs by the formation of iron oxides. Painting to an appropriate specification will significantly extend the period to the onset of corrosion but the durability of the paint finish is largely dependent on the quality of the surface preparation. Improperly installed insulation can provide ideal conditions for corrosion and should be weatherproofed or otherwise protected from moisture and spills to avoid contact of the wet material on equipment surfaces. Application of an impervious coating such as bitumen to the exterior of the pipework is beneficial in some circumstances.

Cathodic protection is an electrochemical method of corrosion control that has found widespread application in the protection of carbon steel underground structures such as pipelines and tanks from soil corrosion. The process equipment metal surface is made the cathode in an electrolytic circuit to prevent metal wastage.

Anodic protection is less commonly used and relies on an external potential control system to maintain the metal in a passive condition. This form of corrosion protection has found practical application in the sulphuric acid manufacturing industry.

Corrosion rates are expressed in terms of inches per year of surface wastage and are used to provide a corrosion allowance in the design thickness of equipment such as vessels and pipework. Operators will often use data based on historical experience from plant operations to aid them in determining appropriate corrosion allowances. Alternatively corrosion charts are widely available that give corrosion rates for many combinations of materials of construction and process fluids and normally a range of values will be provided for various process temperatures. In some instances, particularly where there is a mixture of chemicals present, appropriate data may not exist and corrosion tests may be necessary in order to determine the suitability of equipment. Operators should be able to demonstrate the use of corrosion allowances in equipment specification and design. The sources of data used should be traceable.

Whilst carbon and stainless steels are commonly used materials of construction, increasing use is being made of non- metallic and lined or plastic process equipment. The selection of the material of construction should taken into account worst case process conditions that may occur under foreseeable upset conditions and should be applied to all components including valves, pipe fittings, instruments and gauges. Both composition (e.g. chlorides, moisture) and temperature deviations can have a significant direct effect on the rate of corrosion. The operator should demonstrate that procedures are in place to ensure that potential deviations in process conditions such as fluid temperature, pressure and composition are identified by competent persons and assessed in relation to the selection of materials of construction for pipework systems.

A wide range of plastics are available for use as materials of construction and can be used in areas such as handling inorganic salt solutions where metals are unsuitable. The use of plastic linings is widespread in equipment such as tanks, pipes, and drums. However, their use is limited to moderate temperatures and they are generally unsuitable for use in abrasive duties. Some of the more commonly used plastics are PVC, PTFE and polypropylene.

Special glasses can be bonded to steel, providing an impervious liner. Glass or 'epoxy' lined equipment is widely used in severely corrosive acid duties. The glass lining can be easily damaged and careful attention is required. The thin paint like coatings are unlikely to give full protection due to defects and the most dependable barrier linings are those that are built up in multiple layers to a depth in the region of 3 mm.

Normally testing is carried out in order to determine the suitability of a material of construction for handling a process fluid. However, testing can be used for different purposes. Typically this might be to justify a modified inspection frequency of equipment on an existing plant.

There are a variety of test methods available. Commonly test specimens consisting of small strips or 'coupons' of the material of interest are exposed to the process fluid. The weight loss of the test specimen over a time period is measured in order to determine the corrosion rate. Testing can be carried out on the plant, in the laboratory, or on a pilot plan depending on the situation.

Where laboratory testing is carried out using standard test methods there are difficulties in interpreting results and translating them into plant performance. Care is required to ensure that the test fluid is exactly the same as on the process plant. Discrepancies in test conditions such as trace impurities, dissolved gases, velocity, and turbulence can lead to erroneous results.

Process equipment handling hazardous materials should be inspected at regular frequencies, both internally and externally. Localised corrosion can be unpredictable and fabrication defects such as poor welds can be present. Linings can deform or be damaged. Typically the glass lining on a jacketed reactor can suffer thermal shock or a static discharge may occur through the lining. The frequency of inspection can be amended once an inspection history has been built up and the condition of a piece of equipment can be reasonably predicted. The operator should demonstrate that it has inspection and maintenance programmes in place for hazardous process equipment including lagged systems. Where equipment is lined electrical continuity tests for lining defects should be carried out where appropriate. Cathodic and anodic protection systems should be regularly checked to ensure continued protection.

Where control of corrosion is dependent on the concentration of contaminants or moisture the operator should demonstrate that procedures and the necessary controls are in place to maintain a safe operating condition. Similarly where inhibitors are added or systems such as cathodic protection are used the operator should demonstrate that these systems are inspected and adequately maintained to ensure continued protection of the process.

The flow rate of liquid chlorine through carbon steel pipework is restricted to 2 m/s to avoid removing the ferric chloride coating on the pipe surface which protects against erosion / corrosion of the carbon steel. Wet chlorine gas corrodes mild steel. PVDF (preferably), ebonite, or rubber lined steel is used for this duty. Chlorine gas handled at temperatures in excess of 200°C in carbon steel can result in chlorine / steel fires. Zinc can be used for this duty, but for low temperature (e.g. liquid) chlorine special steels are required to avoid embrittlement. Titanium is unsuitable for Chlorine duty and should be avoided.

Susceptibility of materials of construction to attack by bromine is strongly dependent on the conditions of service including temperature, pressure and moisture content. Therefore, wherever possible materials selected for bromine duty should be tested under the actual conditions of use.

Storage vessels are commonly constructed of steel lined with lead, PVDF (and certain other fluoropolymers) or glass. If the bromine is 'dry' then Nickel or alloys such as Monel and Hastelloy can be used though all are susceptible to severe attack in the presence of wet bromine. Titanium is unsuitable for Bromine duty (wet or dry) and should be avoided.

Lead is used for lining steel storage vessels and less frequently for lining pipes but at high moisture contents and/or elevated temperatures the protective layer of lead bromide that forms on the surface of the metal is susceptible to degradation. Non-metallic linings including glass and certain fluorocarbon polymers, including PVDF and PTFE have replaced lead in most applications. Melt processed polymers such as PVDF, PFA and ETFE are preferred to PTFE due to its inherent porosity.

Few metals are suitable for use in contact with 'wet' bromine (moisture content in excess of 30mg/kg). Niobium, Tantalum and alloys of these two metals are suitable but high cost restricts their use (e.g. bursting discs and instrument components).

Corrosion protection of mild steel vessels occurs by the formation of an iron sulphate coating. Any condition leading to excessive turbulence can result in the removal of the coating and corrosion. Accelerated corrosion can also occur at air/acid interfaces due to interfacial dilution. Additionally the temperature influence on corrosion rate varies with different strengths of acid and consequently it is necessary to define maximum operating temperatures. Chemical lead is sometimes used where steel is unsuitable and PVC or fluorocarbon plastics can be used in certain applications. Specially developed stainless steels have replaced traditional cast iron applications for high temperature duties.

This acid is very corrosive towards most of the common metals and alloys. This is exacerbated where aeration or contamination by oxidising agents is present. Copper is particularly prone to this problem. Also many failures occur due to the presence of minor impurities such as ferric chloride. Plastics and rubber-lined steel are widely used for pipework and small vessels.

Materials of construction for ammonia are dependent on the operating temperature. Whilst mild steel may be used at ambient temperature special steels are required at low temperatures to avoid embrittlement. Impurities in liquid ammonia such as air or carbon dioxide can cause stress corrosion cracking of mild steel. Ammonia is highly corrosive towards copper and zinc.

Bulk storage of 70% acid or greater may be in mild steel or PVDF tanks. Polyethylene, polypropylene, and PVDF are commonly used for construction of major components. PTFE is often used for smaller components such as gaskets. Glass or GRP should never be used.

Materials suitable for liquid oxygen service are nickel steel, austenitic stainless steels, and copper or aluminium alloys. Carbon steels and plastics are brittle at low temperatures and should not be used on liquid oxygen duty. PTFE is the most widely used sealant.

At temperatures below 120°C carbon steel can be used up to high pressures. At elevated temperatures and significant pressures hydrogen will penetrate carbon steel and react with the carbon to form methane. This results in a loss of ductility and cracking or blistering of the steel. For high temperature applications steel alloys containing molybdenum and steel are satisfactory.

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