Corrosion is the deterioration of a material, usually a metal, due to a chemical or electrochemical reaction with its environment. This process naturally transforms refined metals back into more chemically stable forms, such as oxides, hydroxides, or sulfides. General corrosion, also known as uniform corrosion, is the most common form of attack, involving material loss spread evenly across the exposed metal surface. This mechanism of material loss is relatively predictable and leads to a uniform thinning of the component until failure. The most familiar example is the common rusting of steel when exposed to air and moisture.
Understanding the Electrochemical Mechanism
General corrosion occurs through an electrochemical reaction involving the flow of electrons across the metal surface. For this process to occur, three components must be present: an anode, a cathode, and an electrolyte. The metal surface hosts two simultaneous reactions that constantly shift locations, leading to the uniform attack.
The anodic reaction involves metal atoms oxidizing and dissolving into the electrolyte, releasing electrons. This oxidation is the actual material loss, where the metal (M) converts into a metal ion (\(M^+\)) and an electron (\(e^-\)). These released electrons travel through the metal to other sites, which serve as the cathode.
At the cathodic sites, a reduction reaction consumes the electrons, completing the circuit. In neutral or alkaline environments, the predominant cathodic reaction involves the reduction of dissolved oxygen and water to form hydroxyl ions. For corrosion to proceed, an electrolyte (e.g., moisture, water, or soil) must be present to allow ion movement between the anodic and cathodic areas. Since the anodic and cathodic sites are randomly distributed and frequently alternate across the surface, the result is a homogenous loss of material.
Distinguishing General from Localized Attack
The primary characteristic of general corrosion is the uniformity of the attack, resulting in an even thinning of the metal component. Localized corrosion, in contrast, concentrates the degradation at specific points, resulting in deep penetration while leaving the surrounding surface intact. Common forms of localized attack include pitting and crevice corrosion, which can create hidden damage pathways that significantly compromise structural integrity.
Because general corrosion proceeds at a consistent rate across a large area, it is easier to manage and monitor. The widespread attack provides visible signs of deterioration, such as surface discoloration or gradual thinning, long before the component is structurally compromised. Localized corrosion, however, is particularly dangerous because its concentrated damage can lead to sudden, unexpected failure with minimal material mass loss or visible warning. This difference in attack pattern means that general corrosion is highly predictable, while localized corrosion is more insidious and difficult to detect during routine inspections.
Measuring the Rate of Uniform Loss
The predictable nature of general corrosion allows its severity to be quantified using the corrosion rate. This rate is most often calculated based on the mass loss of a material sample over a specific period of time. Engineers use this data to determine the penetration rate, which is the speed at which the metal’s thickness is being reduced.
In engineering contexts, this penetration rate is typically expressed in units of distance per unit of time, such as millimeters per year (mm/yr) or mils per year (mpy). One mil is equivalent to one-thousandth of an inch, making the mpy unit common in the United States. The calculation often involves a formula that uses the measured weight loss, the exposed surface area of the metal, the exposure time, and the metal’s density. A corrosion rate of around 1 mpy is often considered normal for open water systems, while rates exceeding 20 mpy suggest a fast rate of deterioration that requires immediate action. This mass-loss measurement is a reliable method for general corrosion because the attack is uniform, but it becomes highly inaccurate and misleading when applied to localized forms like pitting.
Practical Strategies for Mitigation
Controlling general corrosion involves implementing strategies that isolate the metal from its environment or disrupt the electrochemical reactions. A common defense is the application of protective coatings, which create a physical barrier between the metal surface and the corrosive medium. Organic coatings, such as paints and epoxies, are frequently used, while metallic coatings like galvanizing use zinc to preferentially corrode, thereby protecting the underlying steel.
Another effective approach uses corrosion inhibitors, chemical substances added to the environment or coating formulation. These inhibitors reduce the rate of corrosion by forming a protective film on the metal surface, which effectively breaks the chemical reaction. Finally, material selection plays a large role in mitigation, where inherently resistant materials, such as specific stainless steels or other alloys, are chosen for environments known to be aggressive. In addition, engineers often specify a “corrosion allowance,” which is extra material thickness added to a component to account for the expected, measurable uniform loss over its intended lifespan.