Crevice corrosion can be driven by differences in which two concentrations?

• A. Temperature and humidity
• B. Oxygen concentration and metal ion concentration
• C. Pressure and flow
• D. Light and color

Answer: (B)

Crevice corrosion happens when a confined space creates its own local chemistry, forming driving concentration gradients. In a tight crevice, oxygen cannot diffuse in as quickly as it’s consumed, so the interior becomes depleted of dissolved oxygen compared with the outside solution. That difference sets up an electrochemical cell: the metal surface inside the crevice tends to dissolve (anodic behavior) while the area exposed to higher oxygen outside the crevice acts as the cathode. As the metal dissolves, metal ions accumulate inside the crevice, raising the concentration of dissolved metal ions there and driving further chemical changes (often lowering the local pH and sustaining the attack). So, the two concentrations that drive this corrosion are the oxygen concentration gradient and the concentration of dissolved metal ions inside the crevice. Other factors like temperature, humidity, pressure, flow, or light don’t create the same localized electrochemical gradients that produce crevice corrosion.