Galvanized steel provides a durable defense against corrosion for various industrial and construction applications. The steel is coated with a layer of zinc, which extends its life compared to bare steel. However, galvanized steel is not immune to decay, and the underlying iron will eventually develop the familiar reddish-brown rust. Understanding why this protected material fails involves looking at the specific mechanisms designed to prevent corrosion and the environmental factors that compromise this defense.
How Galvanization Prevents Rust
Galvanization utilizes hot-dipping, where steel is submerged in molten zinc, creating a metallurgically bonded coating. This coating consists of multiple zinc-iron alloy layers beneath an outer layer of pure zinc, forming a durable barrier. The inner alloy layers provide resistance to physical damage.
The main defense mechanism is sacrificial protection, or cathodic protection, provided by the zinc. Zinc is more electrochemically active than iron, meaning that when both metals are exposed to an electrolyte, the zinc will corrode preferentially. This ensures that the zinc sacrifices itself to protect the steel, even if a small area of the base steel is exposed through a scratch in the coating.
Upon atmospheric exposure, the pure zinc outer layer reacts with oxygen and carbon dioxide to form a dense, insoluble zinc carbonate patina. This patina is the primary long-term protector, acting as a passive film that slows the corrosion rate of the zinc itself. The formation of this stable layer is responsible for the decades of protection galvanized steel provides in most outdoor environments.
The Inevitability of Zinc Layer Depletion
The protection offered by the galvanized coating is finite because the zinc must continuously corrode to protect the underlying steel. This sacrificial depletion means the zinc layer is gradually consumed over time, and once exhausted, the base steel becomes vulnerable to rust. The initial thickness of the coating is the primary factor determining the lifespan of the material.
The rate at which the zinc coating depletes is highly dependent on the surrounding environment. In benign rural environments, the loss rate can be as low as 0.5 to 2 micrometers per year, allowing the coating to last for many decades. Conversely, in highly aggressive settings, such as heavy industrial areas or within one kilometer of a coastline due to chloride ions, the depletion rate can accelerate to 15 to 50 micrometers per year.
These aggressive environments accelerate the consumption of the zinc layer because pollutants, such as chlorides and sulfates, interfere with the formation of the protective zinc carbonate patina. These corrosive agents create water-soluble zinc compounds that are easily washed away by rain or actively break down the passive film. Therefore, the higher the concentration of atmospheric moisture and pollutants, the shorter the expected service life of the galvanized coating.
Accelerated Failure Modes of the Protective Coating
Galvanized steel can rust prematurely when its protective mechanisms are compromised.
Mechanical Damage
One common mode of failure is mechanical abrasion or damage, where continuous wear removes the entire zinc-iron alloy structure. This scraping immediately exposes the base steel to the elements, bypassing the sacrificial protection mechanism and allowing localized rust to begin.
Chemical Attack
Chemical attack represents another threat, as the zinc coating is susceptible to rapid dissolution outside a specific pH range. Zinc performs best in solutions with a pH between 5.5 and 12.5. Exposure to highly acidic substances (below pH 5.5), such as acid rain, or highly alkaline solutions (above pH 12.5), like strong cleaning agents, causes the zinc corrosion products to become water-soluble.
The rapid solubility of zinc in these extreme pH conditions prevents the stable, protective patina from forming, leading to faster erosion of the coating layer. Chemical environments with a pH below 3 or above 13.5 are generally not recommended for bare galvanized steel due to the speed of coating destruction. The presence of dissolved salts, agitation, or high temperatures can further compound this chemical degradation.
Galvanic Corrosion
A third form of accelerated failure is galvanic corrosion, which occurs when galvanized steel is placed in direct contact with a more noble metal in the presence of an electrolyte. Metals such as stainless steel, copper, or brass are cathodic to zinc. When these dissimilar metals are connected, the zinc is consumed at an accelerated rate to protect the other metal, leading to localized coating failure and subsequent steel rust at the connection point.
Repairing and Extending the Life of Galvanized Steel
Once rust is observed, several steps can be taken to repair the damage and extend the lifespan of the steel. The first step involves surface preparation, which requires removing all existing rust, loose zinc corrosion products, and contaminants from the damaged area. This ensures that any repair material can adhere properly to the underlying surface.
The most common repair method involves applying a zinc-rich paint or cold galvanizing compound to the affected area. This specialized paint must contain a high concentration of zinc to restore the sacrificial protection mechanism. The compound is applied in sufficient coats to achieve a required thickness, often a minimum of 100 micrometers, comparable to the original galvanization layer.
Other, more specialized repair techniques include:
- Thermal zinc spraying, which applies molten zinc to the damaged area.
- Using a low-melting-point zinc alloy rod for soldering small defects.
To mitigate future accelerated failure, isolate galvanized steel from dissimilar metals by using non-conductive barriers, such as plastic washers or gaskets, at connection points. Regular cleaning to remove corrosive deposits and periodic reapplication of protective coatings also help ensure the longevity of the steel.