Did you know that over 30% of marine equipment breakdowns are caused by corrosion? These issues are not just costly—they're adding up to significant economic burdens. Recent estimates suggest that seawater corrosion costs are now around 4% of a nation's Gross National Product (GNP). Unfortunately, seawater isn't a simple medium—it's chemically and biologically active, leading to a wide range of corrosive impacts. But here's some good news: effective protection against seawater corrosion is possible.
To choose the right material, you'll need to consider several factors, including the strength, damage tolerance, reliability, safety, and longevity required for your specific project. Additionally, environmental variables like chloride concentration, temperature, oxygen levels, and biological activity must be taken into account. These variables are constantly changing and can influence each other, making the selection process complex.
- Chloride Levels: One of the most aggressive causes of seawater corrosion is the concentration of chloride ions, commonly referred to as salinity. Factors like evaporation (which raises salinity), precipitation (lowering it), and dilution can all impact chloride levels. High chloride concentrations can exacerbate pitting corrosion, especially in stagnant water.
- Temperature: The temperature of seawater varies significantly, ranging from 28.4°F at the poles to 95°F in tropical regions. Warmer temperatures accelerate corrosion due to enhanced chemical reactions. Applications involving hot engines or located in tropical areas may experience higher corrosion rates.
- Oxygen Levels: Oxygen plays a crucial role in controlling the rate of corrosion in seawater. Increased dissolved oxygen levels can lead to higher corrosion rates.
- Biological Activity: Biofilms can impact localized corrosion and propagation rates, particularly at temperatures below 30-40°C. The effects depend on factors like crevices, surface deposits, welds, aeration, exposure duration, and flow conditions. Marine organisms and organic matter are more prevalent in coastal areas and near the surface, where light facilitates plant growth.
Materials for Seawater Corrosion Resistance
While there are numerous materials that can help mitigate seawater corrosion, this article will focus on three options that we believe are the most effective and economical for marine environments.
Monel – Ideal for Rough Seas and Fast-Flowing Water
Monel is exceptionally resistant to fast-flowing seawater, with pitting and cracking generally only occurring in stagnant conditions. Its ability to prevent bio-fouling and microbial-induced corrosion makes it a durable and cost-effective alloy for applications requiring wave protection. However, when using Monel alongside steel, iron, zinc, or aluminum, caution is advised since these materials (not Monel) are prone to galvanic corrosion in saltwater. Monel is widely used in offshore platforms and power plants that rely on seawater for cooling.
Titanium – Lightweight and Highly Corrosion-Resistant
Titanium has long been recognized as a top choice for resisting seawater corrosion, remaining virtually unchanged even after years of exposure in marine environments. This durability is due to the formation of a protective oxide layer, TiO2, which forms through the oxidation of the metal. Titanium is resistant to chlorine-containing liquids and can withstand temperatures up to 500°F (260°C). Tests have shown that it can last nearly two decades in polluted seawater. Whether at great depths or exposed to tidal currents, titanium remains impervious to pitting and crevice corrosion. It also exhibits excellent erosion resistance, even at high velocities of up to 120 feet per second.
AL-6XN – A Versatile Solution for Strength and Formability
Developed within the last 30 years, AL-6XN was specifically designed to combat seawater corrosion. Not only is it cost-effective compared to other alternatives, but it also excels in strength, weldability, and formability in saltwater environments. Its tensile strength surpasses stainless steel 316 by 75% and outperforms copper-nickel alloys by over double under ASME standards. Unlike traditional steels, AL-6XN resists stress corrosion cracking (SCC) in chloride environments and is also protected against microbial-induced corrosion (MIC), maintaining corrosion resistance up to +0.5 VSCE. Common applications include reverse osmosis desalination units, power plant service water systems, and seawater heat exchangers.
With the right materials and understanding of the challenges posed by seawater corrosion, protecting your marine equipment becomes both achievable and sustainable.
Changzhou Yingda New Material Co., Ltd , https://www.yingdaspc.com