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Blog Q & A Picture Perfect Corrosion
Picture Perfect Corrosion
Q & A
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I am buying a house where the buyer disclosed a water leak that has since been repaired. He claims that they hung a picture in the home in mid 2011 and a nail must have grazed a water line in the wall causing corrosion. Is this possible? Should I be concerned about mold? Amy in Encinitas
DEAR AMY: The potential issues here can be quite complex. Let's look at these in turn, starting with galvanic corrosion, then we will talk about the potential for mold growth from this water leak. The hope is that following this discussion all involved parties will be a better position to ask intelligent questions and hopefully receive necessary answers.
 
Galvanic corrosion (some times called dissimilar metal corrosion) is the process by which materials in contact with one another and oxidize or corrode. The same chemical process occurs in zinc oxide batteries. There are three conditions that must exist for galvanic corrosion to occur:
  1. There must be two electrochemically dissimilar metals present.
  2. There must be an electrically conductive path between the two metals.
  3. There must be a conductive path for the metal ions to move from the more anodic metal to the more cathodic metal.
Is important to keep in mind that if any one of these three conditions above does not exist, galvanic corrosion will not occur.
 
Now some assumptions. In this case, the crown molding was probably applied with aluminum brad nails from a pneumatic nail gun. Another assumption is that the water supply lines in the home are made from copper. This sets the stage for two dissimilar metals come into contact with one another if the aluminum brad nail grazed the exterior surface of the copper supply line. In the scenario, conditions one and two above would be satisfied. Now for condition three, a water source would be required. So this would imply that there was an active water leak (could have been slight) from the onset. Again, these are only assumptions.
 
Why the scenario is unlikely:
  1. The issue of galvanic corrosion occurring between copper and aluminum however is atypical. For instance, heat transfer coils (which are components of modern air-conditioning systems) around the world are built with aluminium fins mechanically fixed onto a copper pipe, and they all work very happy for many years on the roofs of buildings and in various other environments, albeit without corrosion.
  2. Usually the corrosion occurs in the "less noble" of the metals. In this case, the aluminum would be the one that would suffer from the effects of galvanic corrosion.
Biological organisms such as mold can become viable and thrive in a matter of days provided the environment is conducive to mold growth. Many common molds can grow on organic surfaces when moisture levels reach approximately 20%. The work in this home was done 6 months (potentially) before the leak was discovered. The chances are very good that mold was found and removed during remediation activities.
 
I would recommend advising your clients to ensure that they receive all paperwork related to remediation efforts. All involved parties should have been fully licensed, bonded and insured during renovations. 

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Darin Redding
Written on Friday, 03 February 2012 19:26 by Darin Redding

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Last Updated on Friday, 03 February 2012 20:06
 

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