Originally Posted by Trevor43
The electrolytic action with air and humidity in an empty HW tank.........can you explain that?......a friend was asking.
This may explain it better than I can. I also believe in an oz of prevention and better safe than, well you know. In any case do what works for you.
What causes galvanic corrosion?
For galvanic or dissimilar or electrolytic corrosion to occur, three conditions must be met:
Wetting the join
- The metal join must be wet with a conductive liquid
- There must be metal to metal contact
- The metals must have sufficiently different potentials
The conductive liquid or electrolyte could be rainwater or even water from condensation. The greater the conductivity the more severe the galvanic effects. Salt or industrial pollution significantly increases the conductivity of water so galvanic effects are normally more severe near the coast or in heavy industrial areas. Low conductivity, pure rainwater will only cause slight galvanic effects. One complication is that during evaporation, water films become more conductive so initially benign water may cause quite active galvanic effects as the liquid in the crevice under a bolt or clamp becomes more concentrated. Water may be excluded by design or the use of adhesive sealants.
Metal to metal contact
Galvanic corrosion can only occur if the dissimilar metals are in electrical contact. The contact may be direct or by an external pipe or wire or bolt. If the dissimilar metals are insulated from each other by suitable plastic strips, washers or sleeves then galvanic corrosion cannot occur. Paint is not a reliable separator from direct contact although painting all of the noble metal is quite effective. Painting the active metal causes drill holes at coating defects.
All metals dissolve to some extent when they are wetted with a conductive liquid. The degree of dissolution is greatest with active or sacrificial metals such as magnesium and zinc and they have the most negative potential. In contrast, noble or passive metals such as gold or graphite are relatively inert and have a more positive potential. Stainless steel is in the middle although it is more noble than carbon steel. T