Corrosion in piping systems can cause wall thinning, pitting, leakage and ultimately failure of pipes carrying water, steam, gases, chemicals and other process fluids. Corrosion may occur on the internal pipe surface due to the fluid being transported, or externally due to exposure to moisture, soil, marine conditions or industrial environments.
For engineers, corrosion control is an important part of piping system design because the pipe material, process fluid, temperature, flow conditions, environment and system geometry can all influence the rate and type of corrosion that occurs.
This page provides an introduction to selecting piping materials and corrosion-control methods. It is intended to help students and engineers understand the factors that should be considered when designing or assessing a piping system.
Begin by identifying the fluid being transported and the operating conditions. Candidate pipe materials can then be considered for their compatibility with the fluid and external environment. The system design should also be checked for features that may encourage corrosion, such as poorly drained sections, and whether additional protection such as coatings, linings or cathodic protection is required.
The material examples and tables on this page should be used as general engineering guidance rather than final material-selection data. Actual corrosion resistance can depend on factors including temperature, concentration, pressure, flow velocity, contaminants and operating conditions. Final material selection should therefore be verified using appropriate standards, corrosion data and supplier information.
There is an extremely wide range of pipeline
materials available . These are selected
on various criteria the two most important of which
are suitability for service and cost. The
suitability for service is determined primarily on
the materials resistance to attack by the fluid being
transferred and the external environment.
The internal corrosion resistance is more important
than the external resistance. It is easier to protect
and monitor and repair the external surfaces.
Also the environment generally provides a
less arduous regime.
A typical range of fluids with suitable pipeline
materials is listed below;
| Fluid | Pipe line material |
| Process Air | Carbon Steel, Copper, Plastic |
| Potable Water | Carbon Steel, Copper, Plastic |
| Low Pressure Sat.Steam | Carbon Steel, Copper |
| High Pressure Dry Steam | Carbon Steel, |
| Demineralised Water | 304 Stainless Steel, |
| Seawater | 304/316 St.steels, Aluminium Bronzes, Copper nickel alloys, Nickel alloys, Superduplex, 6%(or7%)Mo St. steels, Duplex steels, Titanium |
| Nitric Acid | 304 Stainless Steel |
| Nitrogen, | Carbon Steel, |
| Argon | Carbon Steel, |
| Instrument Air | Carbon Steel, |
The choice of pipe material is one of the most important factors in controlling corrosion. Different materials provide different levels of resistance depending on the fluid, operating temperature, external environment and other service conditions.
Carbon steel contains no principal alloying elements and is one of the most widely used piping materials. It is relatively inexpensive, readily available and covered extensively by piping design codes. Where corrosion is expected, additional wall thickness may be specified as a corrosion allowance. The suitability of carbon steel depends strongly on the process fluid, temperature and operating environment.
Low alloy steels contain relatively small quantities of alloying elements such as chromium, nickel, molybdenum or vanadium. These additions can improve properties such as strength and high-temperature performance compared with carbon steel. The corrosion resistance depends on the particular alloy and service environment, so low alloy steel should not automatically be considered corrosion resistant.
Steels specifically alloyed for corrosion resistance generally with chromium levels above 18%. Steels resist oxidisation and specific corrosion of virtually all chemicals over a wide temperature range (-200oC to 900oC ). The corrosion resistance is related to the grade selected. Stainless steels are represented in all design codes and are convenient to use. Stainless steel generally costs a least 4 times more than carbon steels.
Two types of cast iron are used, grey cast iron and ductile iron. The former includes graphite as flakes and the latter includes graphite in spherical or nodular form. Cast iron piping has been widely used over the years for transferring a wide range of fluids as it is a low cost option with excellent corrosion resistance to a wide range of environments and fluids. The grey cast irons are generally being superseded by the ductile iron options and the ductile iron piping systems are now being generally replaced by plastic piping. The British Standard for Ductile piping and fittings is BS 4772.
Lead has been used in the past for a wide variety of domestic, civil and chemical piping. It is suitable for most chemicals it is readily available and easily worked. Although lead is expensive it is can be totally recovered and reused. Lead has low strength properties and suffers from creep. It can be alloyed to improve the strength and creep resistance. Lead is no longer used for domestic piping for human health reasons. Lead not widely used now because plastic piping provides improved properties at lower costs.
Piping and tubing made from these metals are used for many purposes throughout industry and also for building and domestic use and for ship pipework engineering because of the resistance of the brasses to sea water attack. Copper and the associated metals generally conform to the relevant standards and are therefore conveniently produced and installed.
Aluminium piping is supplied in two grades to BS 1471: 1972 . Aluminium is used in many industries and provides excellent corrosion resistance compared to steel for arduous fluid flow applications.
Glass piping and glass lined piping provides ideal corrosion resistance to most fluids. However this material is not coveniently installed and has various mechanical limitations. It is used mainly for specialised application including laboratories.
This material has only recently been available in quantity. At this time it is relatively expensive compared to most other materials. However if lifetime costing is consided it would likely be competitive as it has superb corrosion resistance especially for seawater transfer duties. When installed in seawater systems titanium piping provides long continuous service compared to virtually all other metals.
This option provides a relatively low cost method of producing a highly corrosion resistant piping system with good mechanical properties. However if system design, manufacture and installation is not good the lining system may fail resulting in high rates of local corrosion and expensive early repairs.
Piping systems made from plastics including HDPE, ABS, PVC etc. are not subject to the same corrosion problems experienced by metal piping systems. More and more piping systems are being manufactured from plastics However plastics have severely limited mechanical mechanical and thermal properties and are attacked by some chemicals.
This area of design is discussed on separate page Surface coatings
The life of a piping system can be significantly improved if the system is correctly designed in respect to geometry and location. The piping should be designed to ensure that there are no low points which are not fully drained. The piping should also be located to minimise the risk of attack from the environment. If possible piping should be protected against environments which include excessive precipitation in industrial or marine areas.
Galvanic anode system
Buried ferrous or cast iron piping, however well
protected cannot be fully isolated from the moist
soil and will therefore be at risk of corrosion.
Corrosion will occur where the base metal comes into
contact with the salts in the ground with the
presence of water. The base metal will
behave as an anode. Cathodic protection
involves forcing the buried pipe to become a cathode
relative to a buried electrode which will act as an
anode. This involves connecting the pipe
using an insulated cable, to a buried bar made from
magnesium or other metal with a similar electric
potential relative to the pipe material.
As the buried electrode is electrically positive
compared to the pipe this forces the pipe to be a
cathode. The buried bar acts as a
sacrificial anode. The pipe therefore
acts as a cathode and is not corroded.
The sacrificial anodes are buried about 3m from the
pipe and are pitched at about 250m. The
anodes are generally engineered to have a life
expectancy of upto ten years.
Impressed Current System
A similar level of protection can be provided by
applying a generated DC voltage across the pipe and a
buried buried electrode from cast iron or some
similar low cost metal which can discharge a current
to the soil. The applied voltage forces
the buried electrode to be positive relative to the
pipe ensuring that it will behave as an anode
Cathodic protection systems require specialised design involvement.
Corrosion in piping systems occurs when the pipe material reacts chemically or electrochemically with the fluid being transported or with the surrounding environment. The rate and type of corrosion depend on factors including the pipe material, fluid composition, temperature, flow conditions and external environment.
Corrosion can be reduced by selecting suitable pipe materials, applying protective coatings or linings, improving piping system design and, where appropriate, using cathodic protection. The most suitable method depends on the pipe material, service conditions and corrosive environment.
Different piping materials have different levels of resistance to particular fluids and environments. Carbon steel, stainless steel, copper alloys, titanium, plastics and lined piping systems therefore have different applications. Material selection should consider the process fluid, temperature, pressure and expected corrosion mechanisms.
A corrosion allowance is additional pipe wall thickness provided to compensate for material expected to be lost through corrosion during the intended service life of the piping system. The required allowance depends on the anticipated corrosion rate and design requirements.
Cathodic protection is an electrochemical method used to reduce corrosion by making the protected pipeline the cathode of an electrochemical cell. Protection may be provided using sacrificial anodes or an impressed-current system.
Protective coatings and linings can isolate the pipe material from a corrosive environment and significantly reduce corrosion. Their effectiveness depends on correct material selection, surface preparation, application and maintaining the integrity of the protective layer.