Sodium bicarbonate is a mild, white crystalline powder frequently used in household cleaning applications. Corrosion is the chemical process of metal degradation, typically involving oxidation, where the metal reacts with its environment to form an undesirable compound like rust. The general consensus is that baking soda is not corrosive to the majority of common metals under typical cleaning conditions. Its utility comes from its gentle abrasive texture and its chemical properties when dissolved in water, which allows it to neutralize acids and lift grime.
The Chemistry of Low Corrosivity
When sodium bicarbonate dissolves in water, it forms a slightly alkaline solution, generally possessing a pH around 8.3. This mild alkalinity is a significant reason for its low corrosivity on most metals, such as stainless steel and cast iron. The protective oxide layers that naturally form on many metals are stripped away by highly acidic or highly alkaline chemicals.
Baking soda functions as a buffer, meaning it resists dramatic changes in pH. This buffering capacity helps stabilize the environment on the metal surface, preventing the pH from reaching the extreme levels necessary to rapidly dissolve the metal’s protective film. In some industrial applications, bicarbonate ions are intentionally added to water systems to help form a corrosion-inhibiting film on exposed metals like steel and copper. For iron-based metals, the mild alkalinity can also help prevent flash rusting that might otherwise occur with plain water.
Sensitive Metals and Environmental Factors
Despite its generally safe nature, baking soda is not universally non-corrosive, especially when specific conditions or sensitive metals are involved. Aluminum is the most notable exception because it is an amphoteric metal, meaning its surface oxide layer can be degraded by both strong acids and strong bases. The mild alkalinity of a concentrated baking soda paste can attack this protective aluminum oxide layer, leading to discoloration, etching, or pitting over time.
Corrosion can be accelerated by environmental factors such as high temperature and prolonged contact time. When hot water is used with baking soda, the chemical reaction rate increases, and the solution can become more aggressive toward some metals, including steel. Allowing a concentrated paste to sit on a metal surface for extended periods can cause localized damage, particularly on aluminum, where the solution dries and concentrates the alkaline effect. Furthermore, the granular texture of the powder itself can cause mechanical abrasion, which can scratch softer metals like antique silver or gold-plated items.
Safe Practices for Cleaning Metal
To use baking soda safely on metal surfaces, it is important to control the concentration and contact time. Creating a thin paste by mixing the powder with minimal water is often recommended for cleaning, but this paste should not be left on sensitive metals for hours. Test the baking soda mixture on a small, inconspicuous area of the metal first to check for any discoloration or surface reaction.
The most important step after cleaning is to ensure thorough rinsing and complete drying of the metal. Any residue left behind, especially in crevices or seams, can continue to react with the surface or attract moisture, potentially leading to localized pitting or a white powder residue. Rinsing with clean water and immediately drying the surface with a soft cloth will prevent prolonged chemical interaction or water-spotting.