With over 30% of marine equipment failures being the result of marine corrosion, the subsequent costs have added up; now seawater corrosion costs are estimated at 4 % of the GNP. Unfortunately, sea water is not a simple medium because it is chemically and biologically active, which can have an enormity of effects.
The Good News: Protection Is Possible
To select the right material, you’ll first need to take into account the strength, damage tolerance, reliability, safety and longevity you need for your project. In addition, you must also consider chloride concentration, temperature, oxygen, and biology which can be challenging as these variables are ever changing and can affect one another.
- Chloride: The level of chloride ions in salt water, also referred to as salinity, is one of the most aggressive causes of seawater corrosion. Many things affect the salinity including evaporation (increasing saline levels), precipitation (decreasing), and dilution (decreasing). The chlorides can also increase pitting corrosion in stagnant water.
- Temperature: Seawater temperature varies greatly from 28.4 F at the poles, up to 95 F in the tropics. As for any type of water, the warmer it is, the higher the attack of corrosion due to the fact that heat enables the oxide reaction. As a result, applications where there are hot engines or are located in tropical areas will see a higher rates of corrosion.
- Oxygen: The rate of corrosion in sea water is largely controlled by oxygen diffusion. Corrosion rates will increase with heightened dissolved oxygen concentration.
- Biosphere: Biofilms in particular affect local corrosion as well as the propagation rate at temperatures below 30-40°C. The effects of the biofilm is also dependent on the existence of crevices, surface deposits, welds, aeration, longevity of seawater exposure and flow conditions. Organic matter and marine organisms are also more prominent in coastal areas and closer to the surface where light can reach, hence enabling plant growth.
Materials for Seawater Rescue
There are a vast enormity of materials which can assist in helping to prevent sea water corrosion, but in this paper we are going to focus on the three we feel are best suited to effectively and economically operate in marine environments.
Monel – Resistant in Rough Waves & Rapidly Flowing Seawater
Monel is extraordinarily resistant to rapidly flowing seawater, with pitting and cracking typically only occurring in a marine environment if the saltwater is stagnant. This characteristic, combined with its ability to prevent bio-fouling and microbial induced corrosion, makes it a durable, cost-effective alloy to use in applications that need wave protection. Though Monel is extremely resistant to seawater, be cautious when utilizing it alongside steel, iron, zinc and aluminum, as these metals (not Monel) will corrode because of the problem of electrolytic action in salt water (also known as Galvanic corrosion). Monel is a popular choice for applications such as off-shore platforms, as well as power and process plants which use seawater as a coolant.
Titanium – Ideal When Light Weight Is Needed
Titanium has for many years been a gold standard for salt water corrosion resistance, being almost completely unaffected by marine environments. Experiments have been repeatedly performed where titanium was utilized in seawater and sea air situations for years at a time with no change at all. This corrosion resistance is due to the formation of a thin film which is formed by the oxidation of the metal referred to as TiO2. This material is resistant against any liquids containing chlorine. Titanium is resistant to seawater up to as high as 500F (260C) and has been shown to last almost two decades in tests of polluted seawater. It has been used up to a mile in depth and has shown no signs of pitting or crevice corrosion, even after years of exposure. It has even been shown to resist the agitated effects of tides and currents. In terms of high velocity marine environments, titanium is practically completely resistant to any erosion even up to a velocity of 120 ft./sec.
Designed within the past 30 years, AL-6XN was specifically formulated to combat seawater corrosion – its sole developmental goal. It is not only economical compared to other options, but it is the ultimate performer when it comes to strength, weldability, and formability in saltwater environments. It’s tensile strength demonstrates 75% higher allowable stresses than stainless steel 316 and more than twice as much for copper-nickel alloys by ASME standards. Also unlike steels, AL-6XN is resistant to SCC (stress corrosion cracking) in chloride. It is also resistant to microbial induced corrosion (MIC) which is common in a biosphere like the ocean – in fact it is preserved against corrosion to about +0.5 VSCE. Common uses include reverse osmosis desalinization units, power plant service water systems, and seawater heat exchangers.