Galvanic Corrosion - Damage Mechanism # 53


Description:

Galvanic corrosion is a form of electrochemical corrosion that occurs when two dissimilar metals come into contact with each other in the presence of an electrolyte, such as saltwater or acidic solutions. The result is a chemical reaction that causes one metal to corrode while the other remains intact. This type of corrosion is also known as bimetallic corrosion.
This also occurs when two different metals are joined in the soil, such as steel and copper. Electrical current will flow from the steel into the soil and back into the copper resulting in corrosion of the steel. A less recognized but similar phenomenon occurs when new steel is connected to old steel in the soil, such as when replacing a section of corroded pipe. The new steel that is not cathodically protected will frequently experience a higher corrosion rate.

Affected Material:

Galvanic corrosion can affect any two dissimilar metals that come into contact with each other in the presence of an electrolyte, especially in a marine environment where saltwater is present. The likelihood and severity of corrosion depend on the relative position of the two metals in the galvanic series (Ref .1), which ranks metals based on their relative tendencies to corrode. The further apart two metals are in the series, the greater the potential for galvanic corrosion when they come into contact.

Some common metals that are susceptible to galvanic corrosion include:
  • aluminum
  • copper
  • brass
  • zinc
  • Carbon Steel
Aluminum has a very low electrical potential compared to other metals, making it highly susceptible to galvanic corrosion when it comes into contact with other metals such as steel or copper. Similarly, copper and brass are also prone to corrosion when they come into contact with other metals, particularly those with a higher electrical potential. Zinc is another metal that is susceptible to galvanic corrosion, especially when it comes into contact with stainless steel or other alloys with a higher electrical potential.

Some metals are more resistant to galvanic corrosion due to their position in the galvanic series.
  • Stainless steel
is less susceptible to galvanic corrosion than many other metals due to its high chromium content, which creates a protective oxide layer on the surface of the metal. 
  • Titanium 
is also relatively resistant to galvanic corrosion due to its low reactivity and position in the galvanic series.

Critical Factors:

  • Position of joined materials in Galvanic Series
  • Exposed Area between two metals
  • Surface area of the dissimilar metals in contact
  • Nature of electrolyte
The further apart two metals are in the series, the greater the potential for galvanic corrosion when they come into contact. Lower the distance between tow metals, higher will be the corrosion and vice versa. The larger the surface area of contact, the greater the potential for corrosion. The presence of an electrolyte is also critical in the development of galvanic corrosion. Saltwater, for example, is a potent electrolyte that can accelerate the corrosion process. Acids and other corrosive substances can also act as electrolytes and contribute to galvanic corrosion.

Affected Equipment:

Galvanic corrosion can occur in any equipment made of dissimilar metals.
  • It is particularly problematic in marine and offshore equipment such as boats, ships, oil rigs, and pipelines.
  • Heat exchangers are susceptible if the tube material is different from the tubesheet and/or baffles, particularly if saltwater cooling is utilized.
  • Buried piping and ship hulls are also typical locations for galvanic corrosion.

Appearance of Damage:

The appearance of galvanic corrosion damage can vary depending on the metals involved and the severity of the corrosion.
  • Common signs of damage include pitting, rusting, and discoloration of the metal surface. In severe cases, the metal may become weak and brittle, leading to cracking and failure of the equipment.
  • Damage occurs where two different materials are joined at welded, bolted, or rolled connections.

Prevention:

  • Preventing galvanic corrosion requires careful selection of materials and the use of protective coatings and sacrificial anodes. 
  • Materials with similar electrochemical potentials should be used wherever possible, and protective coatings can be applied to the metal surfaces to prevent contact with the electrolyte.
  • Sacrificial anodes made of a more reactive metal can also be installed to corrode instead of the equipment called Cathodic Protection
  • For piping, use specially designed electric insulating bolt sleeves and gaskets

Inspection and Monitoring:

Regular inspection and monitoring of equipment is essential for detecting and preventing galvanic corrosion. 
  • Visual inspections can identify signs of damage, while regular testing of the electrical potential between dissimilar metals can help detect potential problems before they cause significant damage. 
  • VT may indicate loss of the more anodic material by displaying oxidized material before it is cleaned.
  • UT thickness measurement techniques have been effective in determining the amount of loss in the more
  • anodic material.
  • Regular maintenance and replacement of sacrificial anodes can also help prevent corrosion damage.

Summary:


Galvanic Series: