Stainless steel (SS) is a durable, corrosion-resistant material widely used in kitchens, hospitals, and industry worldwide for cookware, water bottles, and storage containers. Consumers often question whether this metal leaches into water or food. Understanding the material’s composition and the conditions under which it interacts with liquids is key to determining its safety.
What Makes Up Stainless Steel
Composition
Stainless steel is primarily an iron alloy defined by the inclusion of chromium, which must be present at a minimum of 10.5% by mass. Food-grade alloys, like 304 stainless steel (18/8), typically contain 18% chromium and 8% nickel. Nickel is included to enhance the steel’s structure and corrosion resistance.
The Passive Film
Chromium is the most important factor for the material’s “stainless” quality. When exposed to oxygen, chromium instantly reacts to form a microscopically thin, dense layer of chromium oxide on the surface. This layer, known as the passive film, acts as a protective barrier, preventing underlying metals from reacting. If the passive layer is damaged, the chromium quickly reforms the oxide, making the material self-healing under normal conditions.
Is Stainless Steel Safe for Everyday Use
Under typical, everyday circumstances, stainless steel is extremely stable, and metal leaching is negligible. When storing plain water or other neutral liquids, the passive chromium oxide layer remains intact and non-reactive. Studies involving neutral pH liquids, such as tap water, show no detectable leaching of metals like chromium or nickel. This stability extends to common uses like boiling water or using standard kitchen utensils. Even at high temperatures, metal release into neutral water is minimal and within acceptable safety limits, making food-grade stainless steel a preferred material.
Specific Conditions That Encourage Leaching
While stainless steel is generally non-reactive, specific conditions can compromise the passive layer and encourage the release of trace metals. The most significant factor is prolonged contact with highly acidic substances, which chemically degrade the chromium oxide film. Low pH liquids, such as vinegar, lemon juice, and tomato-based sauces, increase the rate of metal migration. This effect is intensified when acidic liquids are exposed to the surface for extended periods or at high temperatures. New stainless steel items also tend to release slightly higher amounts of metal initially, though this decreases significantly after the first few uses.
Understanding the Health Effects of Trace Metals
The primary trace metals that may leach from stainless steel are nickel and chromium, and their health effects depend on the amount and chemical form consumed. Chromium is of particular interest because it exists in two common forms: trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)). Trivalent chromium is considered an essential trace element for human health, playing a role in the metabolism of fat, protein, and glucose. The metal that leaches from stainless steel is predominantly Cr(III), which is not associated with the toxicity of the carcinogenic Cr(VI) form. Nickel is a common allergen, and while the amount leached is far below systemic toxicity levels, oral exposure can be a concern for the approximately 10% of the population with a known nickel sensitivity.