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Valves & pipeline materials - Cause of corrosion and erosion

Galvanic corrosion is a major challenge for any pipes which carry sea water. Rust is a particular corrosion problem for steel pipes exposed to contact with sea water or moisture generally and air. Pipe runs along tank tops or on deck, are examples of the latter. Steel pipes in these areas require external as well as internal protection.

Sea water is an electrolyte and therefore a conductor of electricity, because the molecules of its dissolved salts split into positive and negative ions which are available as current carriers. Electrolytic action can result if there are different metals or even differences in the same metal in a pipeline. Galvanic corrosion can occur if the different metals are connected electrically and mutually in contact with the sea water.

A corrosion cell formed between steel and brass in contact with sea water results in wastage of the less noble steel. A list is given in the galvanic series, in which the more noble metals are placed in order after the less noble thus: zinc, aluminium, carbon steels, cast iron, lead—tin alloys, lead, brass, copper, bronze, gunmetal, copper-nickel iron, monel metal. A metal in contact with one occurring later in the series, as with steel and brass, may corrode rapidly in sea water. Because the action is galvanic, less noble sacrificial anodes can give protection.


Steel being subject not only to galvanic corrosion but also to rusting, appears to be a poor material to select for sea water pipes or for installation in tank top or deck areas. Mild steel pipes for sea water are protected by being galvanized or rubber lined. Welding and pipe bending should be completed before galvanizing or application of a lining, so that weld spatter and deposits from manufacture can be removed. The mild steel, electric resistance welded (ERW) or hot rolled pipes are galvanized by hot dipping. Inadequate protection of steel, results if there are pinholes or discontinuities in protective linings. Linings should always be carried over the flange faces.

Mild steel welded fabrications, similarly lined, are also used for large ship side fittings. Seamless mild steel is used for steam, high pressure air, feed discharges and all oil fuel pressure piping. Its strength reduces however, at about 460°C and above this figure, steels require small additions of alloying materials such as molybdenum and chromium. Flanges are secured to steel pipes by fusion welding or by screwing and expanding.

Cast iron

Cast iron has poor corrosion resistance in sea water, being especially vulnerable to graphitization. This form of attack gradually removes the iron from the surface in contact with sea water to leave soft, black graphite. The weakness of ordinary grey cast iron in tension and under shock loading limits its use to low pressure applications, and the brittle nature of ordinary grey cast iron excludes its use for side shell fittings where failure could result in flooding of the machinery space. Ease of casting makes the material ideal for the production of fittings and fortunately techniques for improving strength have been developed.

Spheroidal graphite cast iron (SG iron) and meehanite are examples of high strength versions of the material. These are suitable for use in ship side valves if made to specification by an approved manufacturer. SG iron may be used for high pressure services and for steam below 461°C. Cast iron with its high carbon content and consequent low melting temperature is ideal for the production of fittings by casting.


Copper pipes are suitable for moderate pressures and temperatures. Flanges are secured to copper and its alloys -by brazing or sweating.

Non-ferrous alloys

Basically, brass is an alloy of copper and zinc; bronze an alloy of copper and tin. In both cases there may be additions of other metals and there is some confusion of nomenclature; some high-tensile brasses are called 'bronze' and the practice has prevailed for so long as to be accepted.

Aluminium brass and other non-ferrous pipelines, are considered very resistant to corrosion in sea water, but concentrated galvanic corrosion can occur if some part of the pipe system has a different make up.

A localized corrosion cell can be set up when a fitting, such as a thermometer pocket, is of a brass, bronze or other material which is different to the parent material. Pipe systems are ideally of the same material throughout but non-ferrous alloys are protected against corrosion by the deposition of iron ions so that use of iron or steel fittings is beneficial Iron ion protection can alternatively be supplied from sacrificial or driven iron anodes or by dosing with ferrous sulphate.

Dezincification of brasses is a particular type of corrosion that occurs in the presence of sea water. The attack removes zinc from the alloy, leaving porous copper which is soft. The problem is marked by a patch of copper colour in the brass. Dezincification is inhibited in brasses which are intended for sea-water contact by additions of a very small amount of arsenic (0.04%) or other elements.

Some brasses are prone to corrosion-stress cracking but this is a phenomenon associated chiefly with brass tube which has been stressed by expanding or by being worked in the unsoftened condition and which is also in contact with corrosive fluids, such as sea water. Splitting can occur suddenly, or even violently as a result of stress corrosion cracking.

Stainless steel

A different problem is presented by corrosive liquids and those that contain hard particles and are therefore likely to cause erosion. These can cause differing rates of wastage in conventional metal pipes or cargo tanks. With some corrosive liquids wastage is slow enough, lasting over a period of years, to permit the use of common metals. Expensive stainless steel is widely used for the cargo pipes of chemical tankers intended for carriage of very corrosive cargoes.


Erosion of metal may be the result of abrasives or of high water speeds, entrained air, turbulence and cavitation. The latter are often caused by protuberances, tight bends or an abrupt change of pipe cross sectional area. Erosion from turbulent flow and cavitation also aids corrosion (corrosion/ erosion) by removing the oxide film that assists in the protection of metal surfaces.

The exposed metal surfaces can form galvanic corrosion cells with adjacent areas where oxide film is still present. Erosion is reduced by limiting speed of flow, avoiding sharp bends, changes of section and impediments to flow such as incorrectly cut jointing or weld deposits. Speed of liquid flow should be no greater than 1 m/s for copper; 3 m/s for galvanised steel and aluminium brass; 3.5 m/s for 90/10 cupro-nickel: 4 m/s for 70/30 cupro-nickel.

Summarized below some of the basic procedure of machinery valves & pipeline systems :
  1. Valves & pipeline materials corrosion & erosion

  2. Galvanic corrosion is a major challenge for any pipes which carry sea water. Rust is a particular corrosion problem for steel pipes exposed to contact with sea water or moisture generally and air. Pipe runs along tank tops or on deck, are examples of the latter. Steel pipes in these areas require external as well as internal protection.....

  3. valves-&-pipelines-strength-of-materials

  4. The strength of materials used for pipes and fittings must be adequate for the system pressures and possible over-pressures. Pipelines and valves, for example, used to carry and control the flow of high temperature, high pressure steam must obviously be made to very exacting specifications by approved manufacturers.....

  5. Valves & pipelines-system cleaning & draining

  6. It is often found, in new ships, that the bilges and bilge systems have not been thoroughly cleaned with the result that wood, nuts, bolts, rags and other debris are found inside valves and pipes after initial bilge pumping. These choke the valve-chests and prevent the valves from being properly closed. They also block strainers. ....

  7. Expansion arrangements

  8. Provision must be made in pipe systems to accommodate changes in length due to change of temperature, and so prevent undue stress or distortion as pipes expand or contract.....

  9. Valves & cocks

  10. Cocks and valves are designed to control or interrupt flow. This is done in cocks by rotating the plug, and in valves by lowering, raising or rotating a disc in relation to a seating surface or by controlling the movement of a ball. ...

  11. Butterfly valves

  12. A butterfly valve consists basically of a disc pivoted across the bore of a ring body having the same radial dimensions as the pipe in which it is fitted.....

  13. Gate valves

  14. Unlike the globe valve, gate (or sluice) valves give full bore flow without change of direction. The valve disc known appropriately as a gate,....

  15. Globe valves

  16. The globe valve has a bulbous body, housing a valve seat and screw down plug or disc arranged at right angles to the axis of the pipe....

  17. Relief valves

  18. Excess pressure is eased by a relief valve . This consists of a disc held closed by a spring loaded stem. The compression on the spring can be adjusted so that the valve opens at the desired pressure. ....

  19. Valves traps

  20. A steam trap is a special type of valve which prevents the passage of steam but allows condensate through. It works automatically and is used in steam heating lines to drain condensate without passing any steam. ....

  21. Flap valves & valve chest

  22. Scupper pipes from accommodation spaces are fitted with non-return valves. Those scuppers from spaces below the bulkhead deck, are required to be fitted with non-return valves which can be positively closed from above the bulkhead deck or, if this is not practical, with two non-return valves.....

  23. Quick closingvalves

  24. Fuel oil service and some other tanks must be fitted with valves that can be closed rapidly and remotely in the event of an emergency such as fire. Wire operated valves are commonly fitted,....

  25. Strainers & filters

  26. The term strainer is sometimes used specifically for a simple device made up with a single layer of coarse gauze, a very coarse wire mesh or a drilled or perforated plate. ...

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