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.
Related Technical Measures Documents are:
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:
Process fluid corrosion
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
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
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
Control of operating conditions
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
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
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
Codes of Practice relating to corrosion
- HS(G)28 Safety advice for bulk chlorine
installations Safety Advice for Bulk Chlorine Installations, HSE,
Briefly deals with the limitations of some materials of construction for
- HS(G)30 Storage of anhydrous ammonia under
pressure in the UK : spherical and cylindrical vessels, HSE, 1986
(Not in current HSE list).
Gives advice for the appropriate materials of construction for ammonia
- RC4 - "Bromine in Bulk Quantities - Guidelines for safe storage
and handling", Chemical Industries Association, 1989.
Includes appendices containing guidance on materials of construction
suitable for bromine duty.
- ASME B31 Guide for piping and piping systems
A comprehensive standard for the design of pipework systems. In section
B31.3 the standard defines categories of hazardous materials that are
then used to define the standard of appropriate piping components.
- GEST 79/82 , 'Choice of materials of construction for use in contact
with chlorine', Euro Chlor.
A typical industry sector standard containing specific guidance on the
use of materials of construction for chorine systems.
Further reading material
- Perry's Chemical Engineers' Handbook, Section 23, Transport and Storage
of Fluids, McGraw Hill.
- Lees, F.P., 'Loss Prevention in the Process Industries: Hazard
Identification, Assessment and Control', Second Edition, 1996.
- 'Corrosion Engineering', MG Fontana, McGraw Hill, 1987.
- Case Studies Illustrating the Importance of Corrosion / Selection of
Associated Octel Company Limited (1/2/1994)