Stainless steel is an iron-based alloy combined with a minimum of 10.5% chromium. This composition allows the material to resist rust, which afflicts plain iron and carbon steel. Stainless steel can generally be submerged in water without rusting, but its resistance depends heavily on the type of water and the specific grade of the alloy used. The long-term viability of the material relies on a chemical process that can be disrupted by certain environmental factors.
How Stainless Steel Resists Water Damage
The inherent rust resistance of stainless steel stems entirely from the chromium in the alloy. When the steel surface is exposed to oxygen, whether from the air or dissolved in water, the chromium immediately reacts to form a microscopically thin layer of chromium oxide. This invisible film, called the passive layer, acts as a continuous, stable barrier protecting the iron underneath from further oxidation.
This protective film is self-repairing; if the surface is scratched or damaged, the exposed chromium quickly reacts with surrounding oxygen to reform the layer. For the steel to remain “stainless,” this passive layer must be maintained and allowed to reform when compromised. This mechanism effectively stops the process that causes traditional iron to corrode and flake away as red rust.
Water Conditions That Cause Corrosion
While the passive layer is resilient, certain water conditions can overwhelm its protective capabilities and lead to localized corrosion. The most significant threat is the presence of chloride ions, which are abundant in saltwater, heavily chlorinated pools, and some municipal water sources. Chloride ions chemically attack and break down the chromium oxide film, creating tiny, localized pits in the metal surface. This phenomenon, known as pitting corrosion, can begin with chloride concentrations as low as 25 parts per million (ppm) in common grades of steel.
Crevice corrosion is another common failure point, occurring in tight spaces where water becomes stagnant and oxygen cannot circulate freely. In areas like under bolt heads, washers, or tight joints, the lack of oxygen prevents the passive layer from repairing itself once damaged. The chemical environment inside these crevices can become more acidic and concentrated with aggressive ions, leading to rapid, localized failure of the steel.
Water temperature also plays a role in accelerating the rate of corrosion. Elevated temperatures increase the chemical reactivity of the water, making it easier for chloride ions to penetrate the passive layer. Temperatures exceeding 60°C (140°F) significantly increase the risk of chloride stress corrosion cracking, especially for common grades like 304 and 316. This corrosion type is dangerous because it can cause sudden, unexpected failure in stressed components.
Understanding Stainless Steel Grades
The term “stainless steel” refers to a large family of alloys, with different compositions conferring varying degrees of corrosion resistance. The choice of grade is the most important factor when selecting metal for a wet environment. The two most common austenitic grades are 304 and 316, which are distinguished primarily by the inclusion of one element.
Grade 304 stainless steel is the most widely used, containing approximately 18% chromium and 8% nickel. It offers good resistance to fresh water and many common chemicals, making it suitable for indoor kitchen sinks, appliances, and general-purpose hardware. However, it is susceptible to pitting corrosion when exposed to even moderate levels of chlorides.
Grade 316 stainless steel is an improvement for wet or corrosive applications due to the addition of molybdenum, typically between 2% and 3%. Molybdenum significantly enhances the passive layer’s resistance to chlorides, making 316 far less susceptible to pitting corrosion than 304. This superior resistance is why 316 is often referred to as “marine grade” and is the standard choice for outdoor architectural applications, coastal environments, and chemical processing equipment.
Maintaining Stainless Steel in Wet Environments
Even the most robust grades require simple maintenance to ensure longevity when exposed to water. Allowing water to evaporate naturally can leave behind mineral deposits and concentrated corrosive agents, like salt, which attack the protective layer. Wiping stainless steel items dry after exposure, especially to chlorinated or saltwater, is an effective way to prevent localized corrosion.
Regular cleaning with warm water and a mild detergent helps remove accumulated dirt, grime, and mineral deposits before they compromise the surface. Avoid using harsh abrasive materials, such as steel wool or scouring pads, as these physically damage the passive film and embed tiny iron particles that will rust. Cleaners containing chlorides should also be avoided, as they directly contribute to the breakdown of the protective layer. After cleaning, rinsing the surface thoroughly and promptly drying it allows the chromium oxide film to quickly reform and maintain its protective barrier.