General corrosion, also known as uniform attack, represents the most common and predictable form of material degradation. This process involves a chemical or electrochemical reaction that causes metal loss to occur evenly across the entire exposed surface area of a material. Unlike localized corrosion types, such as pitting or crevice attack, the resulting material thinning is homogenous, which makes the rate of degradation easier to measure and forecast. The rate of general corrosion is typically expressed as a mean loss of metal thickness over time, often measured in millimeters per year. This predictable nature allows engineers to design structures with an added thickness, called a corrosion allowance, to ensure a specific service life.
The Electrochemical Process of Uniform Corrosion
The underlying mechanism of uniform attack is an electrochemical reaction that requires four simultaneous components to establish a microscopic corrosion cell. These components include an anode (where oxidation occurs), a cathode (where reduction takes place), an electrolyte (a conductive liquid medium), and an electrical connection between the anode and cathode. The metal itself forms the electrical connection, while the electrolyte is typically water containing dissolved ions, such as moisture in the air or a liquid solution.
When an iron surface is exposed to the atmosphere, a thin film of moisture acts as the necessary electrolyte. At random, microscopic anodic sites, the iron metal oxidizes, releasing electrons and forming iron ions. These released electrons travel through the body of the metal to adjacent cathodic sites, which are often areas with higher oxygen concentration.
In a neutral or alkaline water environment, the cathodic reaction involves the reduction of dissolved oxygen. The iron ions generated at the anode react with the hydroxide ions produced at the cathode to form iron hydroxide, which further oxidizes to create the familiar reddish-brown iron oxide, or rust. The uniformity of this corrosion results from the anodic and cathodic sites continuously shifting and alternating across the metal’s surface, ensuring the attack is distributed rather than concentrated in one spot.
Environmental Factors Driving Corrosion Rate
External environmental conditions dictate the speed at which this uniform attack proceeds. Temperature is a significant factor, as higher temperatures increase the reaction kinetics of both the anodic and cathodic processes, leading to an accelerated corrosion rate. However, this effect can be partially offset in open systems because the solubility of oxygen in water decreases as the temperature rises.
The concentration of dissolved oxygen in the electrolyte directly influences the speed of the cathodic reaction, especially in neutral or alkaline solutions. For instance, steel corrodes much faster in aerated water than in deaerated water. The pH level of the environment is another strong determinant, since acidic conditions (low pH) dramatically increase the rate of corrosion by promoting the hydrogen evolution cathodic reaction.
Fluid velocity, the speed of the electrolyte flowing over the metal surface, can also influence the rate of uniform attack. Increased flow can continuously remove any partially protective corrosion products or passive films that may have formed on the metal, thereby exposing fresh metal surface to the corrosive environment. The presence of aggressive ions, such as chlorides from road salt or seawater, significantly increases the electrolyte’s conductivity, accelerating the transfer of electrons between the anode and cathode and thus increasing the overall corrosion rate.
Strategies for Preventing General Corrosion
Preventing uniform attack is a primary goal in materials science and engineering, utilizing several strategies designed to break the required components of the electrochemical cell. A common and cost-effective approach involves the use of protective coatings, which act as a physical barrier to isolate the metal surface from the electrolyte and oxygen.
Organic coatings like paints, epoxy, and polymers are widely used, often in multi-layer systems where an anti-corrosive primer may contain chemical inhibitors. Metallic platings, such as hot-dip galvanization, offer protection by coating steel with a layer of zinc.
Galvanization is a form of sacrificial protection where the zinc coating acts as a sacrificial anode, preferentially corroding to protect the underlying steel even if the coating is scratched. This is an effective method for protecting carbon steel. For applications in highly corrosive environments, material selection is the first line of defense, involving the choice of inherently resistant alloys.
Metals like stainless steel utilize chromium, which reacts with oxygen to form an extremely thin, passive, and self-healing oxide layer that prevents further corrosion. Aluminum and titanium also form stable, protective oxide films, making them suitable for many atmospheric and aqueous environments. In closed systems, such as cooling water loops or pipelines, corrosion inhibitors are chemical additives introduced into the fluid. These inhibitors work by slowing down the anodic or cathodic reactions, such as by forming a protective film on the metal surface or neutralizing aggressive chemical species in the electrolyte.