Steel is a versatile alloy, primarily composed of iron and carbon, used in countless applications from cookware to construction. When discoloration appears on steel items, people often describe it as “tarnish,” but this term is frequently misapplied to iron-based metals. The surface changes observed on steel are typically the result of more aggressive chemical processes like oxidation and corrosion. The true concern for steel is not superficial change, but the material degradation caused by the formation of iron oxide, commonly known as rust.
Tarnish, Corrosion, and Oxidation: Defining the Terms
Oxidation is the broad chemical reaction where a material loses electrons, often by reacting with oxygen. This fundamental process underlies all forms of metal surface discoloration. Tarnish, in the strictest sense, is a mild, surface-level form of corrosion that primarily affects metals like silver and copper. It usually involves a reaction with sulfur compounds, forming a thin film that merely dulls the metal’s appearance.
Corrosion is the gradual deterioration of a material, typically a metal, due to chemical or electrochemical reactions with its environment. It is a specific, destructive type of oxidation that breaks down the material’s structure. Rust is the most specific term, referring exclusively to the reddish-brown, flaky corrosion that affects iron and its alloys, including steel. Rust requires the simultaneous presence of both oxygen and moisture to form iron oxide.
How Different Types of Steel React to the Environment
The primary difference in how steels react chemically depends on the addition of chromium. The two major categories of steel—stainless and carbon—behave very differently when exposed to air and water.
Stainless Steel
Stainless steel contains a minimum of 10.5% chromium. This chromium reacts instantly with oxygen to form an extremely thin, invisible layer of chromium oxide on the surface. This protective layer, known as the passive film, is self-repairing and highly resistant to further oxidation, which is why stainless steel resists rust. Discoloration on stainless steel is rarely traditional rust, but is often heat tint from high temperatures or localized corrosion known as pitting.
Pitting Corrosion
Pitting corrosion occurs when the passive layer is compromised, frequently by exposure to chlorides, such as salt or bleach. Saltwater environments are particularly harsh because the chloride ions attack the protective film, leading to small, deep cavities in the metal’s surface. This results in small dark spots or a greenish film, which signals localized material breakdown.
Carbon Steel
Carbon steel, also known as mild steel, lacks the chromium content necessary to form a durable passive layer. Because its composition is predominantly iron, it is unstable when exposed to the atmosphere. When carbon steel is exposed to oxygen and water, the iron readily oxidizes to form the characteristic reddish-brown iron oxide. This process is destructive because the rust layer is porous and flaky, constantly exposing fresh metal to corrosive elements.
Maintenance and Prevention of Surface Discoloration
Preventing discoloration in steel depends heavily on managing the environment and maintaining the protective surface.
Stainless Steel Maintenance
For stainless steel, the focus is on preserving the integrity of the passive layer. Routine cleaning with mild soap and water, followed by immediate drying, helps prevent corrosive agents from settling. It is important to avoid abrasive materials like steel wool, which can scratch the surface and embed tiny particles of carbon steel that will rust. Avoid prolonged exposure to harsh chemicals, particularly those containing chlorides, such as household cleaners or bleach. In high-salt environments, frequent rinsing with fresh water is necessary to wash away accumulated corrosive salts.
Carbon Steel Maintenance
For carbon steel, prevention requires creating a physical barrier between the metal and the environment. Since it lacks a natural protective layer, a protective coating is necessary to block oxygen and moisture. This is commonly achieved through painting, galvanizing, or applying a thin layer of mineral oil or wax after use for items like knives and tools. This barrier prevents the contact necessary for the destructive oxidation process to begin.