Salt water significantly accelerates the rusting of metal compared to fresh water. Rust is a common form of corrosion that degrades metals. When iron or steel, which are iron alloys, encounter salt water, they degrade at a much faster rate.
Understanding Rust Formation
Rust is the common term for the corrosion of iron and its alloys, such as steel. This process occurs when iron reacts with oxygen in the presence of water, forming iron oxides. The reddish-brown, flaky substance seen on metal surfaces is primarily hydrated iron (III) oxide. Rusting is an electrochemical process where iron atoms lose electrons (oxidation) and oxygen gains them, leading to rust formation.
The Accelerating Effect of Salt Water
Salt water speeds up the rusting process due to its enhanced electrical conductivity. When salt, such as sodium chloride, dissolves in water, it dissociates into ions. These dissolved ions, particularly chloride ions, act as electrolytes, allowing electrons to flow more easily through the water. This increased conductivity facilitates the electrochemical reactions involved in rust formation. Chloride ions can also react directly with the metal, forming soluble iron compounds that further expose the underlying iron surface to oxygen and water, intensifying the corrosion.
Preventing Salt Water Corrosion
Protecting metals from salt water corrosion involves various strategies:
- Applying protective coatings creates a barrier between the metal and the corrosive environment. Common coatings include marine-grade paints, epoxy, powder coatings, or grease and lubricants for moving parts.
- Galvanization, which involves coating steel with a layer of zinc, offers another effective defense. The zinc layer corrodes preferentially, providing sacrificial protection for the underlying steel.
- Sacrificial anodes, typically blocks of more reactive metals like zinc, aluminum, or magnesium, are attached to structures such as boat hulls. These anodes corrode instead of the protected metal.
- Regular maintenance, including rinsing with fresh water to remove salt deposits, also helps mitigate corrosion.
Metals and Salt Water Resistance
Different metals react differently to salt water exposure.
Ferrous Metals
Ferrous metals, like iron and steel, are susceptible to rusting. Stainless steel, an iron alloy containing chromium, resists rusting because chromium forms a thin, protective passive layer of chromium oxide on its surface when exposed to oxygen. This layer acts as a barrier against corrosion. While stainless steel is highly resistant, certain grades, like 316, offer superior resistance to chloride-induced pitting and crevice corrosion, making them suitable for marine applications.
Non-Ferrous Metals
Non-ferrous metals do not rust in the same way as iron, but they can still corrode through other mechanisms. Aluminum, for instance, forms a protective oxide layer, but salt water can still cause pitting corrosion. Copper and brass, an alloy of copper and zinc, generally show good resistance to salt water, but brass can undergo “dezincification” where zinc is selectively removed, potentially compromising its structural integrity. Copper can also form a protective patina over time, but prolonged exposure to salt water can still lead to corrosion. Titanium exhibits exceptional resistance to salt water corrosion, making it a choice for demanding marine applications.