Corrosion of Piping
Corrosion of Piping Notes...
Introduction
A significant proportion of the corrosion problems experienced relate to piping systems. Piping systems are used to transfer a wide range of fluids in a wide range of environments. A major lifetime costs for any industrialised plant is the cost of leaking fluids and the cost of repair of piping system to prevent and eliminate leaks. The main methods of preventing corrosion in piping systems are as follows;
- Selection of materials
- Corrosion resistant surface coating
- Location /Geometry of piping systems
- Cathodic protection methods
Piping Materials
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, |
Pipeline Material Notes
Carbon Steel
Material containing no principle alloying
elements. Piping useable up to 430oC.
Widely used and design requirements are
detailed in all relevant codes.
Corrosion allowed for when necessary by applying
corrosion allowances.
Low Alloy Steel
Material contains small percentages (less than 3%) of
alloying elements such as chromium, nickel,
molybdenum or vanadium. The material has
higher temperature range compared to carbon steel.
Stainless Steel
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.
Cast Iron /Ductile IronTwo 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
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.
Copper, Brass, Copper Nickel AlloysPiping 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.
- Copper tubing is used where ease of fabrication is important.
- 70%/30% - Cu/Zn brass is a good general purpose material used for a variety of applications e.g. heat exchanger tubes, and closed circuit systems.
- Admiralty brass 70% /1%/29% - Cu/Sn/Zn has slighty improved resistance to polluted water compared to 70/30 brass.
- Brass with 76%/2%/0,04%- Cu/Al/As and Remainder Zn has good resistance to seawater attack and is used for diverse process plants for transferring seawater under turbulent conditions to resist corrosion and impingement attack.
- Cupro Nickel Containing 31%/2% - Ni/Fe and " Kunifer" containing 10.5%/1.7% - Ni/Fe are also used for transferring seawater and high good strength at elevated temperatures.
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 and Glass Lined pipingGlass 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.
TitaniumThis 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.
Elastomer/Plastic Lined Steel PipingThis 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.
Plastic Piping SystemsPiping 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.
Surface Coating Options
This area of design is discussed on separate page Surface coatings
Location/Geometry of piping systems
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.
Cathodic Protection
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.
Links to Corrosion of Piping
- National Physical Laboratory ... downloadable guides ..Excellent
- Tubesales Australia Good Concise notes and a corrosion table
- Corrosion Doctors An excellent reference site for the very important topic of corrosion
- Corrosionsource Relevant online magazine