Corrosion in Fasteners

GALVANIC CORROSION - BAD CHEMISTRY FOR CONSTRUCTION

REAL CASE STORY

At a carpentry class for contractors, the topic of galvanic corrosion came up. One of the contractors said, "I just installed a wrap-around deck on a house where I used stainless-steel joist hangers and galvanized nails. Do I have a problem?"

"Yes, you have a big problem", said the instructor.

The problem? Galvanic Corrosion.

Because dissimilar metals were placed in contact with each other, all that is needed is an electrolyte to begin accelerated corrosion. And one excellent electrolyte is water; an even better electrolyte is saltwater.

Stainless steel and the zinc in galvanized steel are very dissimilar and will react to each other. When an electrolyte (like water or saltwater) is present, the stainless steel becomes a negatively-charged cathode, and the zinc in the galvanized steel becomes a positively-charged anode. Electrons flow from the stainless steel to the galvanized steel with the help of the electrolyte. The two metals begin to act as a battery transferring electrons from the stainless steel (the cathode) to the zinc or galvanized steel (the anode).

The electrical movement between the two metals causes the stainless steel to corrode at a slower rate than normal and the galvanized steel to corrode at a faster rate than normal. The following picture shows stainless-steel nuts and bolts in contact with a galvanized-steel beam.

Source: wikiwand.com/en/Galvanic_corrosion

Note how the galvanized beam has corroded at a much faster rate around the stainless steel bolts and nuts. This accelerated corrosion is directly caused by the galvanic reaction between the two metals.

Implication for the decking contractor:

For the contractor who installed stainless-steel joist hangers with galvanized nails, the problem is very serious. The galvanized nails will corrode at an accelerated rate. When the nails are weakened, they will fail to hold up the joist hangers which will fail to hold up the joists. This will likely result in a catastrophic failure of the deck structure.

Either the nails need to be replaced with stainless steel, or the joist hangers need to be replaced with galvanized steel. For this home, replacement of the galvanized-steel nails was the best solution.

IMPLICATIONS FOR MCFEELY'S CUSTOMERS:

When selecting fasteners and other building hardware, do not mix metals shown as red or yellow in the following chart. Be especially aware of the danger in mixing stainless steel with galvanized steel since both of these metals are common in exterior construction. The following chart shows the safety or danger of mixing metals:

Source: Internet - Original author unknown

Red = Most Problematic
Yellow = Problematic
White = Safe

The use of stainless steel and zinc or galvanized steel merits a red score. This combination must be avoided.

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