An alloy is a material made by combining two or more chemical elements, with at least one being a metal, to achieve superior properties. When alloys encounter water, they can degrade, but the speed and nature of this degradation depend entirely on their chemical makeup. The process of metallic degradation in water is broadly known as corrosion. Predicting an alloy’s longevity requires understanding the relationship between its composition and its environment.
Defining Corrosion and Rust
Corrosion is the general term for the deterioration of a material, usually a metal, due to a chemical or electrochemical reaction with its environment. This process is essentially the metal returning to a more chemically stable form, such as an oxide or sulfide, often triggered by water or air. Corrosion can manifest as general surface thinning, localized pitting, or the formation of a surface layer that changes the metal’s color.
Rust is a specific type of corrosion that only affects iron and iron-containing alloys, such as steel. Rust is the reddish-brown substance, chemically known as iron oxide, that forms when iron reacts with both oxygen and water. Water acts as the electrolyte that accelerates this chemical oxidation reaction.
How Alloying Elements Provide Resistance
Alloying elements are intentionally added to a base metal to slow or prevent corrosive reactions. The primary mechanism for resistance is the formation of a protective, self-repairing surface layer called a passive film. This film is typically a thin, non-reactive oxide layer that acts as a physical barrier between the underlying metal and the corrosive environment.
Elements such as chromium are effective because they react rapidly with oxygen to create a dense, stable chromium oxide layer. This passive film effectively seals the metal below, halting the oxidation process. Increasing the content of chromium and nickel enhances protective ability against uniform corrosion. Molybdenum improves localized corrosion resistance, particularly against pitting and crevice corrosion, by forming protective compounds within the passive film. Aluminum and zinc operate similarly, forming tenacious oxide layers that resist further attack from water.
Common Alloys and Their Reaction to Water
The wide variation in alloy composition leads to a vast spectrum of reactions when exposed to water, ranging from nearly impervious to highly susceptible. Highly resistant alloys, such as stainless steel and aluminum alloys, owe their durability primarily to the protective passive layer.
Stainless steel contains a minimum of about 10.5% chromium, which forms the self-healing chromium oxide film that makes it highly corrosion-resistant in fresh water. Performance varies based on the environment; for instance, Grade 316 stainless steel includes molybdenum, which significantly improves resistance to chlorides found in saltwater, making it the preferred choice for marine use. Aluminum alloys also show excellent resistance to fresh water due to their protective oxide layer, but they can be susceptible to localized pitting corrosion when exposed to certain salts or heavy metals.
Moderately susceptible alloys, primarily copper-based materials like brass and bronze, do not rust because they contain little or no iron. Instead, they undergo corrosion that results in tarnishing or the formation of a protective green or blue-green patina layer. Naval brass, alloyed with tin and zinc, improves performance in seawater but still experiences a slower form of corrosion compared to stainless steel.
Highly susceptible alloys like plain carbon steel, which is iron with a small amount of carbon, rust readily when exposed to water. Carbon steel lacks the necessary alloying elements to form a strong passive layer. The iron component quickly oxidizes, forming the characteristic flaky, expanding iron oxide that continually exposes fresh metal to the environment. This rapid oxidation makes carbon steel unsuitable for continuous immersion without external protection.
Protecting Alloys from Water Damage
Several external methods can be used to protect alloys from water damage beyond their intrinsic resistance.
- Surface treatments, such as painting, powder coating, or clear coats, apply a physical barrier that prevents water and oxygen from contacting the metal surface. These coatings must be maintained, as scratches can expose the underlying metal and allow local corrosion to begin.
- Galvanization involves coating the alloy with zinc, which acts as a sacrificial anode. Zinc corrodes preferentially to protect the base metal, even if the coating is slightly scratched.
- Applying a high-quality sealant or ceramic coating creates a durable, hydrophobic layer that repels moisture, particularly useful for components exposed to road salts.
- Regular cleaning with pH-neutral cleaners is important, as contaminants like dirt or salt residues can disrupt the passive layer and accelerate corrosion.