Environmental Factors Affecting Galvanized Coatings

In our last post, we explored the corrosivity of the Australian atmosphere and how local Standards help predict the durability of galvanized coatings. Today, we will dive deeper into how specific environmental factors like moisture, pollution, and proximity to the sea influence the performance of these coatings. Understanding these factors is vital for designing and maintaining structures with optimal longevity.

 

Effect of Atmospheric Conditions

 

Atmospheric Moisture and Carbon Dioxide

The presence of moisture in the air significantly affects the formation and stability of the protective patina on galvanized coatings. Initially, zinc reacts with oxygen to form zinc oxide, which then transforms into zinc hydroxide in the presence of moisture. This layer further reacts with carbon dioxide to form stable zinc carbonates, creating a resilient barrier against further corrosion.

 

Rural vs. Coastal vs. Industrial Areas

1. Rural Areas: In rural environments with low pollution levels, galvanized coatings can last over 100 years. However, micro-environments created by aerial spraying of fertilisers or insecticides can reduce their lifespan. While dry fertilisers and insecticides are harmless, their interaction with water can form aggressive solutions that attack the zinc coating until washed away by further wetting.

2. Coastal Areas: Proximity to the sea increases the corrosion rate due to soluble chlorides in the atmosphere. This leads to the formation of zinc chloride based compounds in the patina, making it moderately soluble in water. Despite this, galvanized coatings perform exceptionally well in coastal areas, especially when used as part of a duplex system (paint over galvanizing).

3. Industrial Areas: The presence of atmospheric impurities such as sulphurous gases and chemicals in industrial areas leads to the formation of soluble zinc salts. These can be washed away by moisture, leading to the continuous exposure of fresh zinc to corrosion. In extremely corrosive industrial environments, galvanized coatings are generally not recommended, unless they are reinforced with a chemical-resistant paint system.

 

Case Studies

1. RMIT Design Hub (C2 Urban Zone): Located in Melbourne, this structure exemplifies how galvanized coatings perform in an urban inland environment with low corrosivity.

2. Gas Processing Facility in NT (C3 Coastal Zone): This facility illustrates the challenges and effectiveness of galvanized coatings in a medium-corrosivity coastal environment.

 

This post highlighted how atmospheric conditions, pollution, and proximity to the sea affect the performance of galvanized coatings. Each environment presents unique challenges that influence the longevity of the protective zinc layer.

In our next and final post of the series, we will examine the performance of galvanized coatings under specific conditions such as high temperatures, underwater environments, and soil.