Fundamentals of Metallic Corrosion in Fresh Water   -   6

Pits in copper are nearly always well isolated, suggesting that, unlike tubercles formed in iron, the tubercle in copper, once formed, continues to be active.

When the pH of water is below 7, the rate of formation of Fe⁺⁺⁺ from Fe⁺⁺ and O₂ is very much slower than at higher pH values. Hence tubercles are less likely to form, so that oxygen concentration cells are less likely to be self-perpetrating, and corrosion is more apt to be uniform.

Values of pH less than 7 are usually encountered in waters of low alkalinity and low dissolved solids. Under these conditions, uniform corrosion results. Corrosion may be severe in terms of total loss of metal, but in the absence of pitting, perforation will not be rapid and facilities often have reasonably long life. Nevertheless, these low-pH, low-alkalinity waters may be highly undesirable for drinking water supplies. Lead pipe and fittings are rapidly attacked and solder from copper pipe joints enters the water. Lead presents a health hazard in concentrations of only a small fraction of a part per million.

In wells, the portions of the casing and column above the water level are usually covered with condensate saturated with air. Since air contains carbon dioxide, the water has a pH of less than 7 and alkalinity and dissolved solids are for practical purposes equal to zero. Well casing subject to this uniform corrosion becomes so thin that failure occurs after periods up to a century.

Under these conditions, the classical corrosion reactions adequately explain the results. The lower pH values favor the reaction:

2H₂O + Fe = Fe⁺⁺ + 2H + 20H⁼

The atomic hydrogen then reacts with oxygen:

4H + O₂ = H₂O

with HO-OH (hydrogen peroxide) as a likely intermediate. Note that these reactions are equivalent to those that have been described for the galvanic cells and the oxygen concentration cell. In all cases iron is oxidized to ferrous ions and electrons. The electrons react with oxygen and water to form hydroxyl ions. The principal difference is that, in uniform corrosion, the cathode and anode are separated by microscopic distances, while in galvanic and oxygen concentration cells, the anode and cathode may be separated by several millimeters.

Dezincification, although it occurs infrequently, is an interesting subject. Yellow brass alloy is replaced by porous copper. It was at first assumed that the zinc was selectively dissolved from the alloy, but it is now generally agreed that the entire alloy dissolves. Copper ions in solution find themselves more noble than the alloy, so they plate the surface, leaving a porous structure having very little mechanical strength. Dezincification occurs only in high zinc alloys, principally yellow brass (67% Zn - 33% Cu), and corrosion is usually caused by oxygen concentration cells or galvanic action. However, zinc is amphoteric and is attacked by hydroxyl ions:

Zn + 20H^- + 1/2 O₂ = ZnO₂ + H₂O

In one large water system where pH was raised to a value greater than 9 by lime softening, operators encountered numerous failures of brass valve stems. Dezincification of these stems was attributed to the above reaction.

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