Year upon year the cost of marine corrosion has increased until it is estimated today at 4 % of the Gross National Product. An enlightened approach to materials selection, protection and corrosion control is needed to reduce this burden of wasted materials, wasted energy and wasted money. These notes have been compiled by Members of the Marine Corrosion Forum to help marine designers, engineers, and equipment users, understand the causes of marine corrosion and the way in which protective systems and more resistant materials can be used to reduce or entirely eliminate sea water corrosion problems.
Many different types of destructive attack can occur to structures, ships and other equipment used in sea water service. The term 'aqueous corrosion' describes the majority of the most troublesome problems encountered in contact with sea water, but atmospheric corrosion of metals exposed on or near coastlines, and hot salt corrosion in engines operating at sea or taking in salt-laden air are equally problematical and like aqueous corrosion require a systematic approach to eliminate or manage them.
Corrosion by sea water, aqueous corrosion, is an electrochemical process, and all metals and alloys when in contact with sea water have a specific electrical potential (or corrosion potential) at a specific level of sea water acidity or alkalinity - the pH.
This typical diagram shows the regions where the metal will freely corrode; the region of passivation where stable oxide or other films form and the corrosion process is stifled; the region of pitting corrosion where the corrosion potential of the metal exceeds that of its oxide; and the region of immunity where the metal is normally fully safe to use. More resistant alloys mean less corrosion, metals like gold platinum and tantalum can resist virtually all corrosion, but for marine service the final choice will always be a compromise with cost.
Most corrosion resistant metals rely on an oxide film to provide protection against corrosion. If the oxide is tightly adherent, stable and self healing, as on many stainless steels and titanium, then the metal will be highly resistant or immune to corrosion. If the film is loose, powdery, easily damaged and non self repairing, such as rust on steel, then corrosion will continue unchecked. Even so, the most stable oxides may be attacked when aggressive concentrations of hydrochloric acid are formed in chloride environments.
Sea water, by virtue of its chloride content, is a most efficient electrolyte. The omni-presence of oxygen in marine atmospheres, sea spray and splash zones at the water-line, and sometimes surprisingly at much greater depths, increases the aggressiveness of salt attack. The differential concentration of oxygen dissolved at the waterline or in a droplet of salt spray creates a cell in which attack is concentrated where the oxygen concentration is lowest. Crevices which allow ingress of water and chlorides but from which oxygen is excluded rapidly become anodic and acidic and are hidden start points of corrosion.
There are five main methods for controlling the tendency of metals to corrode in sea water:
Use of non metallic materials including composites may offer a solution for some applications.
Sea water, if not destructive enough on its own, has several powerful allies assisting the breakdown of metals and non metals alike. Living allies in sea water also enhance its destructive power. Microbiological organisms, clusterings of weed, limpets as well as deposits of sand, silt or slime not only exclude oxygen but often create locally corrosive conditions under these deposits which aggravate attack. Coatings and composite structures can experience rapid degradation. Sulphate reducing bacteria, left undisturbed in marine silt or mud deposits, will produce concentrations of hydrogen sulphide which are particularly aggressive to steel and copper based alloys.
Pitting attack in stagnant sea water may be as much a problem as impingement, erosion or cavitation attack at higher velocities. The highest water velocities, at the tips of propellers or in pumps can result in bubbles of entrained air imploding with sufficient energy to remove metal or break up composites. Called cavitation, this noisy and aggressive mechanical destruction must be corrected by design, or if it cannot be eliminated, countered by the selection of suitably resistant alloys.
High levels of stress in service, or residual stress from manufacturing may result in selective corrosion of more highly stressed regions of an otherwise corrosion resistant structure. In the aggressive marine environment even the more resistant alloys may be affected by hydrogen-induced cracking, or by chloride or sulphide stress corrosion cracking. Choosing the right material for corrosion resistance also requires careful attention to component design, selection of manufacturing processes, installation and operation.
Let's now look at a simple example. A ship made from bare mild steel will quickly rust.
Protection by painting
Painting the ship isolates the steel from the corrosive media. The paint must also be resistant to the marine environment and the application strictly controlled to ensure full and effective coverage of the steel. Regular inspection and repair of the coating may be necessary to achieve reliable and lasting protection.
Sacrificial anodes enable the potential of the system to be changed and will provide temporary protection to steel exposed by wear or damage of the protective coating. Systematic location of the anodes is critical to their overall effectiveness. They must likewise be regularly serviced and replaced when spent.
Inside the ship inhibitors which modify the corrosion process may effectively prevent attack in bilges and other areas where sea water will collect and stagnate. Reliable systems to monitor and maintain the correct concentration of the inhibitor are an essential aspect of this prevention strategy.
In practice ships are rarely made just from a single metal or alloy. Modern engineering systems use a wide range composites and of metals and alloys, some more, some less resistant to marine corrosion than steel. The more resistant alloys may aggravate the attack on adjacent unprotected less resistant alloys. This galvanic effect is not always confined to separate metals, some alloys improperly processed in manufacture or fabrication carry the seeds of their own destruction in their microstructures which contain phases so widely separated in corrosion potential that without further overall protection by coating, anodes or inhibitors, selective attack of the less resistant phase is inevitable.
Using corrosion resistant alloys
Could ships and other marine structures be made from more corrosion resistant materials? Depending on design factors including the severity of the application and the levels of strength, damage tolerance, reliability, safety and life required, components and systems can be manufactured from composites, or from stainless steels of increasing resistance, or from copper based alloys such as cupro-nickel or nickel aluminium bronze, nickel alloys or titanium, using these materials exclusively or in conjunction with each other or less resistant alloys. Protection for the least resistant alloys by anodes, or impressed potential, requires careful control of the system potential to avoid the possibility of hydrogen uptake by the more highly corrosion resistant alloys such as super duplex steel and titanium.
Key factors in prevention of marine corrosion are design, selection of materials, construction, use and maintenance. Failings in any one of these may lead to a total failure to prevent attack, which once started may cost far more to correct or eliminate than any notional savings on materials achieved at the outset. In a recent survey corrosion was found to be responsible for 30% of failures on ships and other marine equipment. These are expensive errors arising from the selection and use of unsuitable materials and are compounded by ever increasing penalties on vessels, civil and military for breakdown and unnecessarily short intervals between outages for major repairs. On offshore platforms the cost penalty for replacement of failed equipment is several times that required for a similar onshore facility, and this does not take into account any losses of oil or gas production.
Where to get help
The many types of marine corrosion, their possible interaction, and the need to review the whole system when considering changes, means that getting help and advice from marine corrosion specialists and materials and coatings experts is particularly important. Members of the Marine Corrosion Forum include such specialists as well as product and systems suppliers and end users. Regular meetings review and update the wide range of options available to designers and users to overcome new or long term marine corrosion problems. You are welcome to attend a meeting as a guest, or to become a member of the Marine Corrosion Forum yourself.