Galvanic Corrosion

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Galvanic corrosion is a concern when ever you have dissimilar metals in the same system. The best thing to do is make sure you have silicate in the system. You can get this from Anti-freeze (they are added in) or from a local Industrial chemical supplier.

I work as an outside operator at a major oil refinery and have seen 1/4 carbon steel pipes eaten out in less then a year. My refinery use Nalco as are chemical supplier. To fight corrosion in my system I use Nalco 2833 for pH and corrosion control.

The other thing you can do is use untreated ground water – this water is already full of minerals and won’t act as a conduit for corrosion cells.

The corrosion mechanism that goes on in water systems, especially with corrosive water (e.g., distilled, de mineralized, deionized, reverse osmosis), is a natural process of the metals returning back to their more stable oxide states. The process is accelerated by heat, oxygen, low pH as well as dissimilar metals.

The corrosion cell that takes place has an anode (where the metal is lost from) and a cathode (where the electrons flow to). Metals have different potentials for corrosion and the metal that has a higher potential becomes the anode. If you have dissimilar metals touching in a system, the one that corrodes easiest (i.e. highest on the galvanic series) will dissolve while the other metal remains intact. The smaller the anode area the quicker and more localized the corrosion becomes.

Without corrosion control, corrosive water will dissolve the anodic metal quickly causing failures. With corrosion control, the galvanic corrosion is slowed down by making the electrolyte less effective in the corrosion cell circuit and by passivating the metal surfaces with their own oxide film to minimize any further deterioration. These disruptions lead to corrosion protection.

In some systems where there is ammonia, oxygen and high pH (above 9.2), there is a special problem with yellow (copper and its alloys) metal corrosion.

Copper can selectively corrode in this environment which is non-corrosive to other metals. This dissolved copper can circulate in the system and plate out in hotter areas on dissimilar metal surfaces. This sets up a corrosion cell where the copper is a cathode to metals that corrode easier like aluminum and iron.

This can result in localized pitting where the plated out copper causes severe losses in small areas of iron or other easier to corrode metal.

ED NOTE: I am using “ZEREX Water Pump Lubricant and Protector” in my systems (contains Sodium Siliconate). It comes in a 15.5 ounce bottle for under $2. I figure a little lubricant will prolong the life of mag drive waterpumps – these typically use an impeller that rotates on a shaft.

Clinton Hansen

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