Understanding Bimetallic Corrosion: Prevention and Protection in Metal Structures

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What is Bimetallic Corrosion?

Bimetallic corrosion is a phenomenon where preferential corrosion occurs between two joined metals of different galvanic status, with the corrosion specifically attacking the anodic metal. This electrochemical process represents one of the most common forms of accelerated corrosion in structural applications, particularly affecting steel structures where dissimilar metals are joined together.

The fundamental mechanism involves an electrochemical reaction that requires the simultaneous presence of moisture and oxygen. During this process, iron in structural steel undergoes oxidation to produce rust, which occupies approximately six times the volume of the original material. This volume expansion can lead to significant structural stress and potential failure if left unaddressed.

Environmental Factors Affecting Galvanic Corrosion

The rate at which bimetallic corrosion progresses depends primarily on the micro-climate immediately surrounding the structure. In atmospheric environments, the electrolyte consists of a thin condensed film of moisture containing soluble contaminants such as acid fumes and chlorides. The formation of this critical moisture film depends on three important variables: temperature, relative humidity, and the presence of atmospheric dust particles.

The characteristics of moisture films on metal surfaces differ significantly from those of bulk electrolytes. In thin moisture films, the replenishment of dissolved oxygen occurs much more rapidly than in bulk electrolyte solutions, owing to the large ratio of surface area to electrolyte volume. Under conditions of reduced relative humidity, which permits rapid evaporation, convective mixing in the condensed layer further accelerates the arrival of dissolved oxygen at the cathode. Both characteristics can result in increased galvanic corrosion rates.

Localized Nature of Atmospheric Bimetallic Corrosion

The electrolyte conductivity of the condensed layer parallel to the metal surface remains low compared with bulk electrolyte, even when containing acid fumes or chlorides. This high electrolytic resistance of the thin condensed electrolytic layer exerts a controlling effect on corrosion distribution. Galvanic attack typically becomes highly localized and rarely extends beyond 25 mm from the bimetallic junction.

The geometrical anode-to-cathode area ratio, which represents an important factor in observed corrosion rates for galvanic couples, has limited influence in atmospheric conditions. The controlling effect of electrolytic resistance applies equally to both anode and cathode components of the bimetallic couple. In atmospheric bimetallic corrosion, the active areas remain small and approximately equal, regardless of the geometrical area ratios.

Galvanic Series and Metal Compatibility

When two dissimilar metals are joined together and contact an electrolyte, electrical current passes between them, causing corrosion to occur on the anodic metal. Some metals, such as stainless steel, cause low alloy structural steel to corrode preferentially, while other metals like zinc corrode preferentially themselves, thereby protecting the low alloy structural steel through cathodic protection.

The tendency for dissimilar metals to undergo bimetallic corrosion depends partly on their respective positions in the galvanic series. The greater the separation between two metals in the series, the greater their tendency toward galvanic corrosion. This fundamental relationship helps engineers predict and prevent potential corrosion issues during the design phase.

Risk Assessment and Environmental Considerations

The nature of the electrolyte significantly influences bimetallic corrosion behavior. Bimetallic corrosion presents the most serious concerns for immersed or buried structures, where continuous electrolyte contact occurs. However, in less aggressive environments, such as stainless steel brick support angles attached to mild steel structural sections, the effect on steel sections remains minimal.

For most practical building or bridge situations, no special precautions are required. However, for higher-risk situations, gaskets, sleeves, and similar electrically insulating materials should be implemented. Alternatively, applying a suitable paint system over the assembled joint provides effective protection.

The tendency for bimetallic corrosion is also influenced by the relative surface areas of the cathodic and anodic metals (Ac/Aa ratio). In simple terms, the greater the Ac/Aa ratio, the greater the tendency for bimetallic corrosion to occur.

Essential Conditions for Bimetallic Corrosion

For bimetallic corrosion to occur, three specific conditions must exist simultaneously. If any one of these conditions is absent, bimetallic corrosion will not begin under any circumstances:

First, there must be two electrochemically dissimilar metals present, though they need not be in direct physical contact with each other. Second, there must be an electrically conductive path between the two metals to allow current flow. Third, there must be an electrolyte present to allow metal ions to conduct along the provided path from the more anodic metal to the more cathodic metal.

Prevention and Protection Strategies

The most effective approach to preventing bimetallic corrosion involves applying dielectric insulating materials between dissimilar metals. When dissimilar plates are bolted together, plastic or rubber washers can effectively insulate the metals from each other. Similarly, dielectric unions separate and insulate dissimilar plumbing components, preventing electrical contact while maintaining structural integrity.

Additional protection strategies include proper material selection during the design phase, ensuring that metals with similar galvanic potentials are used together whenever possible. When dissimilar metals must be joined, protective coatings and regular maintenance schedules help extend service life and prevent premature failure.

Table 1. Corrosion category and risk

Corrosivity category and risk	Low-carbon steel Thickness loss μm	Examples of typical environments in a temperate climate (informative only)
		Exterior	Interior
C1 very low	<1.3	-	Heated buildings with clean atmosphere, e.g. offices, shops, schools, hotels
C2 low	>1.3 to 25	Atmospheres with low level of pollution. Mostly rural areas	Unheated buildings where condensation may occur, e.g. depots, sports halls
C3 medium	> 25 to 50	Urban and Industrial atmospheres, moderate sulphur dioxide pollution. Coastal area with low salinity	Production rooms with high humidity and some air pollution e.g. food-processing plants, laundries, breweries, dairies
C4 High	> 50 to 80	Industrial areas with high humidity and aggressive atmosphere	Chemical plants, swimming pools, coastal, ship and boatyards
C5-I very high (industrial)	> 80 to 200	Industrials areas with high humidity and aggressive atmosphere	Buildings or areas with almost permanent condensation and high pollution
C5-M very high (marine)	>80 to 200	Coastal and offshore areas with high salinity	Buildings or areas with almost permanent condensation and with high pollution