Silo Painting, Nottinghamshire

THE PROBLEM

The stainless steel silo was exhibiting heavy staining and dissimilar/galvanic corrosion where mild steel adjoined the stainless steel. A protective and aesthetic protective coating solution was required.

Why does dissimilar metal corrosion occur?

Dissimilar metal corrosion, also known as galvanic corrosion, occurs when two different metals come into contact in the presence of an electrolyte (such as water, especially saltwater). The fundamental cause lies in the electrochemical differences between the metals. Here’s a breakdown of the process:

1. Electrochemical Differences:

• Electrochemical Series: Different metals have varying tendencies to lose electrons (become oxidized). Metals can be ranked in the galvanic series, with those at the top (like magnesium, zinc) being more anodic and prone to corrosion, while metals lower down (like copper, gold) are more cathodic and resist corrosion.
• When two dissimilar metals are in electrical contact and exposed to an electrolyte, the more anodic metal (higher in the series) will act as the anode and corrode, while the more cathodic metal (lower in the series) will act as the cathode and be protected from corrosion.

2. Formation of a Galvanic Cell:

• Electrolyte: An electrolyte is necessary to facilitate the flow of ions. In the presence of moisture (especially salty or polluted water), an electrolyte is formed.
• Anode and Cathode: The metal higher in the galvanic series (anode) will lose electrons, while the cathode (lower metal) remains protected. This sets up a galvanic cell, with the electrolyte allowing ions to move and carry current between the metals.

3. Electron Flow:

• Electrons flow from the anodic metal (which corrodes) to the cathodic metal. This electron flow drives the corrosion process.

4. Environmental Factors:

• The rate and severity of galvanic corrosion depend on factors like:
  • Difference in electrode potential between the two metals.
  • The area ratio of cathode to anode: a large cathode relative to a small anode accelerates corrosion on the anode.
  • The presence of aggressive electrolytes like saltwater, which increases conductivity and corrosion rates.

Example:

When steel (anodic) is in contact with copper (cathodic) in a moist environment, the steel will corrode while the copper remains largely unaffected. This is because steel is higher up in the galvanic series and thus more prone to oxidation.

How to Prevent Dissimilar Metal Corrosion:

1. Isolate Metals: Use insulating materials (like plastic or rubber) between the two metals to prevent electrical contact.
2. Select Similar Metals: Where possible, use metals that are close together in the galvanic series.
3. Apply Protective Coatings: Coatings such as paints or galvanizing can prevent the electrolyte from reaching the metal.
4. Cathodic Protection: Use sacrificial anodes (like zinc) to intentionally corrode instead of the more important metal.

In summary, dissimilar metal corrosion occurs because of the electrochemical potential difference between two metals when in contact with an electrolyte, causing one metal (the anode) to corrode while the other (the cathode) is protected.