Galvanized steel is a common material used across many industries, from construction to automotive, due to its enhanced resistance to corrosion. This material is known for its durability, particularly where bare steel would quickly degrade. However, despite its protective qualities, galvanized steel can still develop rust over time. Understanding the mechanisms behind its protection and eventual failure helps clarify why this process occurs.
Understanding Galvanized Steel
Galvanized steel is steel coated with a layer of zinc, primarily to prevent rust. The most common method is hot-dip galvanizing, where steel is immersed in a bath of molten zinc. During this process, the zinc metallurgically bonds with the steel, creating a durable, multi-layered coating. This zinc layer provides protection through two main mechanisms.
First, zinc acts as a physical barrier, shielding the underlying steel from direct exposure to corrosive elements like moisture and oxygen. Second is sacrificial protection, also known as cathodic protection. Zinc is more reactive than steel, meaning it will corrode preferentially to protect the steel, even if the coating is scratched or damaged. This sacrificial action ensures the steel remains protected as the zinc layer slowly depletes.
The Chemistry of Rust
Rust is a form of corrosion specific to iron and its alloys, such as steel. It is an iron oxide, typically reddish-brown, formed through an electrochemical process. For rust to occur, three components must be present: iron, oxygen, and water. Water acts as an electrolyte, facilitating the transfer of electrons necessary for the reaction.
The process begins when iron loses electrons to oxygen, a reaction accelerated by the presence of water. This oxidation results in the formation of iron oxides and hydroxides, collectively known as rust. Unlike the protective oxide layer that forms on some metals, iron rust is porous and flaky, allowing corrosion to continue deeper into the material. Without moisture, the rusting process would be significantly slower or might not occur at all.
When Galvanization Fails
While the zinc coating offers substantial protection, it is not permanent, and galvanized steel can eventually rust. The primary reason for this is the gradual degradation of the zinc layer itself. Through sacrificial protection, the zinc actively corrodes to shield the steel, leading to its slow depletion over time. This natural process of zinc depletion is an expected part of its protective function.
Physical damage, such as scratches or impacts, can also compromise the zinc layer, directly exposing the underlying steel. Once the zinc coating is sufficiently thin or breached, the steel beneath becomes vulnerable to the rusting process. The protective zinc carbonate patina that forms on the surface, which initially provides an additional barrier, can also degrade under certain conditions, further reducing the coating’s effectiveness.
Factors Accelerating Corrosion
Several environmental and mechanical factors can significantly accelerate the degradation of the zinc coating. High humidity levels, especially above 60%, increase the corrosion rate of the zinc layer. Exposure to saltwater or marine environments drastically speeds up zinc corrosion due to the presence of chlorides, which can lead to the formation of soluble zinc compounds. Industrial pollutants, such as sulfur dioxide, can also create acidic conditions that rapidly consume the zinc coating.
Temperature fluctuations, particularly continuous exposure to temperatures above 392°F (200°C), can cause the outer free zinc layer to peel from the underlying zinc-iron alloy layers, accelerating the degradation process. Galvanic corrosion can occur if galvanized steel is in direct contact with more noble metals like copper or stainless steel in the presence of an electrolyte. In such scenarios, the zinc will corrode at an accelerated rate to protect the other metal, hastening the overall failure of the galvanized coating.