Marine environments pose an obvious challenge to the lifetime of metals, with multiple forms of corrosion and failure possible. Seawater is an efficient electrolyte that facilitates the electrochemical reaction of corrosion. The presence of chloride ions accelerates the reaction, together with the presence of dissolved oxygen. In addition, there can be an influence from the speed of water flow, local pollution and microbiological activity.
Corrosion mechanisms
General corrosion
For corrosion resistant alloys, general corrosion is not normally a consideration. The alloy additions made ensure that a very thin, passive layer is formed on the surface during its manufacture and subsequent exposure to the environment. Therefore, corrosion typically only occurs where this passive layer is damaged or removed.
Pitting corrosion
The most immediate risk of corrosion comes from pitting. The passive layer may be physically damaged and then unable to re-passivate if immersed in seawater. Alternatively, steelmaking impurities, known as inclusions, could be present at the metal surface and prevent the formation of an effective passive layer.
Or the passive layer can be chemically damaged due to localised conditions. Once pitting has started, it can progress rapidly given the creation of a small anode (within the pit) compared with a large cathode (the remaining metal surface) accelerating the electrochemical reaction.
Increasing erosion resistance
In general, increasing the chromium, molybdenum and nitrogen content will increase a stainless steel's resistance to pitting (and crevice) corrosion. This can be simply summarised as the Pitting Resistance Equivalent number (PREN), calculated as:
PREN = %Cr + 3.3 × %Mo + 16 × %N
Alternative formulas also include tungsten (W) as a beneficial alloy addition. From Langley Alloys experience of super duplex stainless steels, increased additions of Cu can also be helpful in limiting the impact of pitting corrosion.
It is also possible to measure the Critical Pitting Temperature (CPT) through controlled experiments, to indicate the effect of temperature on the likelihood of pitting corrosion. The CPT is found by exposing metal samples in an aggressive 6% FeCl3 / 1% HCl solution, typically for up to 72 hours. The temperature is progressively raised until pitting is observed at a specified magnification/incidence.
Both the PREN and CPT will give an indication of resistance to pitting, so are useful as a basic reference, but give no information on their actual behaviour in a live environment.
Crevice corrosion
Narrow gaps around a component can lead to different environmental conditions compared with the bulk i.e. more acidic, or typically oxygen-depleted. This can initiate corrosion, which can then proceed at an accelerated rate in a similar manner to pitting corrosion. Crevice corrosion can be limited through careful design of components, ensuring there is uniform flow around a component to avoid such conditions occurring.
Erosion corrosion
Here, the impact of high flow rates or abrasive media in the water can lead to the partial removal or damage of the protective passive layer. At this point corrosion can begin. Continued removal of both the passive layer (if it attempts to re-form) or the products of corrosion (which can slow down subsequent corrosion) are removed, leading to more general and continued corrosion occurring.
Cavitation corrosion
Cavitation is the formation of vapour cavities within a liquid, usually when it is subject to rapid changes in pressure. The cavities form in areas where the pressure is lowest, and these voids effectively 'implode' and can damage the passive layer on the metal surface at this point. Cavitation can commonly occur around propellers, pump vanes and seals and piping and is designed-out by limiting the pressure changes within a system.
Hydrogen embrittlement
Hydrogen embrittlement has been historically associated with the presence of hydrogen in steel from its initial production. However, another source of hydrogen is from poorly controlled cathodic protection systems used in marine applications.
Hydrogen diffuses along the grain boundaries and combines with carbon, which is alloyed with the iron, to form methane gas. The methane gas collects in small voids along the grain boundaries where it builds up enormous pressures that initiate cracks. Hydrogen embrittlement is not a permanent condition - if cracking does not occur and the environmental conditions change so that no hydrogen is generated on the surface of the metal, the hydrogen can re-diffuse from the steel, restoring ductility.
Stress corrosion cracking
As the term suggests, this form of failure depends upon the co-existence of both corrosive conditions and stress (either applied or residual from the fabrication process). Their combined effect results in relatively rapid failure in a discrete part of the component, as cracks can quickly propagate (usually along grain boundaries). The stress experienced by a part can be limited by careful design, avoiding sharp corners or other 'stress concentrators'.
Sulphide stress corrosion cracking
This form of failure combines elements of both hydrogen embrittlement and stress corrosion cracking. In environments where hydrogen sulphide (H2S) is present, such as in oil & gas applications, the metals react with H2S to generate hydrogen, which is then absorbed into the metal. For applications in such sour environments, there is a considerable level of materials data available from the NACE MR 0175 specification.
Microbiologically induced corrosion
Metals immersed in water may be subject to the colonisation by micro-organisms, resulting initially in a thin biofilm before progressing to larger deposits. Ultimately the surface may become colonised by macro-fouling, such as shell-fish and weeds. The composition of the deposit is significantly different from and more corrosive than the bulk water environment as concentration cells and differential aeration occurs.
Materials selection
Materials for marine applications are selected to maintain the integrity of the structure (mechanical properties such as strength, hardness and impact) and to be corrosion resistant. Stainless steels are commonly used because they combine resistance to corrosion, are easily fabricated and offer good mechanical properties.
Austenitic stainless steels
Standard austenitic grades such as Alloy 316L and its derivatives are suitable for coastal service environments, splash zone applications and intermittent submersion in seawater. Once described as "marine grade" stainless steel, they are no longer recommended for permanent contact with seawater. Applications include handrails, housings for equipment, ladders and ship deck components such as deck eyes, brackets for anchor ropes, shackles and rails.
Fermonic® 50
Fermonic® 50 is a nitrogen-strengthened austenitic stainless steel, providing almost twice the yield strength of standard grades. It is therefore more suited to load bearing applications such as chain plate pins and supports for rigging in yacht applications - the increased strength allowing designers to reduce the section thickness and weight of the components for equivalent or higher performance.
Fermonic® 60 is a more specialised alloy designed to provide superior galling resistance. For components where there is sliding or rotating contact it can often be used without additional lubrication - ideal for marine applications where there will be repeated assembly and disassembly.
Duplex and super duplex stainless steels
Such alloys provide a competitive alternative to the higher performance austenitic grades, combining high levels of pitting and crevice corrosion resistance with higher strength levels. Standard duplex grades are based upon 22% Cr with a PREN of approximately 34. Super duplex stainless steels based upon 25% Cr, such as Ferralium®, have PREN > 40, making them credible alternatives for more expensive nickel alloys in selected applications.
Although the level of chromium addition is higher, providing superior pitting resistance, duplex grades utilise lower additions of the more expensive nickel and molybdenum elements, so can be very cost effective. The only downside with duplex grades is the degree of care needed when welding to avoid the creation of undesirable phases.
Compared with other duplex grades, Ferralium® is less susceptible to the formation of sigma phase. In addition, the inclusion of copper helps retard the creation of pitting. Common marine applications include shipbuilding propellers, shafts, rudders, shaft seals, pumps, bolts, fasteners, valves and instrumentation.
Offshore structures present different requirements depending upon whether their application is topside, splash zone or subsea. Ferralium® 255 - SD50 is widely used for bolting and fasteners, matching B7 carbon steel strengths but with far superior corrosion resistance and a service life equal to the life of the system.
In the splash zone, it has demonstrated its suitability for seawater resistance with over 15 years' service on North Sea installations, and has been widely employed for riser bolting and riser protection system components on platforms.
Copper-nickel alloys (cupronickels)
Such alloys are widely used for marine applications due to their excellent resistance to seawater corrosion, low macro-fouling rates, and ease of fabrication. The addition of nickel to copper improves strength and corrosion resistance while allowing the alloy to remain ductile.
Hiduron® 130
Hiduron® 130 is a high-strength cupronickel, where aluminium additions result in significant precipitation strengthening. Being a cupronickel, it has excellent resistance to corrosion by seawater and in marine and industrial atmospheres and it is highly resistant to crevice corrosion. The major use of Hiduron® 130 is in subsea hydraulic and electrical connectors such as flying lead connectors for stab plates. It is also used in naval winches, seawater valves and marine engineering.
Hiduron® 191
Hiduron® 191 is an update on the original Hiduron® 130 product. Although the yield strength is somewhat reduced, the ductility and impact resistance are significantly improved. It has been widely used in naval and defence applications for fasteners for ships and submarines, control elements of submarines, pump shafts, mechanical seals, winch components, gears, hose couplings, weapons handling equipment, components in sonar devices, propeller shafts and drive bushes.
Hidurel® 5
Hidurel® 5 is a copper-nickel-silicon alloy combining high electrical and thermal conductivity with very good notch ductility and high mechanical strength. Resistance to corrosion under marine and industrial conditions is excellent and the alloy has good anti-frictional and bearing properties. With a magnetic permeability of less than 1.001, the alloy is essentially non-magnetic. Applications have included shafts and bolting for ships and submarines, flanges, swash plate pump components and mine detection equipment.
Nickel alloys
Nickel alloys have excellent corrosion resistance including resistance to pitting and sulphide stress corrosion cracking, along with high strengths and retention of mechanical properties over a wide range of temperatures. They are inherently more expensive than other metals and are therefore most commonly considered for demanding applications in oil and gas where their performance advantage can be fully justified.
