Water continued zinc coatings

Kruse et al. (1983) tested zinc tubes and steel pipe specimens with different zinc coatings. All specimens were corroded in Dortmund tap water in 11 test lines containing hot water at approximately 60°C and cold water, with both discontinuous and continuous flow in certain tests there were occasional additions of polyphosphates or orthophosphates. One test line was operated with warm water from a water heater, with cathodic protection of the reservoir by the Guldager method. Visual inspection, as well as the deter-... 

There is evidence that surface coatings containing zinc metal can adsorb and retain iodine from water at a high pH (9 -10). Continued retention of the iodine requires that the water in contact with the zinc coating be maintained at a high pH. Hence, for this mitigation strategy to be effective, it must be combined with the use of pH buffer to control the pH of the water in containment. 

Conclusion It was concluded that iron was severely corroding. The amount of iron increased overnight when the water was relatively unused. In the morning, when the taps were opened, a substantial amount of iron leached in the water rendering it unsuitable for use. The reason was the reversal of polarity which occurs in hot water, zinc became the cathode and iron the anode. The zinc coating did not, therefore, protect the steel pipe which continued to corrode by becoming anode. 

The primary function of a coating is to act as a barrier which isolates the underlying metal from the environment, and in certain circumstances such as an impervious continuous vitreous enamel on steel, this could be regarded as thermodynamic control. However, whereas a thick bituminous coating will act in the same way as n vitreous enamel, paint coatings are normally permeable to oxygen and water and in the case of an inhibitive primer (red lead, zinc chromate) anodic control will be significant, whilst the converse applies to a zinc-rich primer that will provide cathodic control to the substrate. 

The only way we can get the oxidation of the metal to continue is to couple it with some other process that restores electroneutrality to the two phases. A simple way to accomplish this would be to immerse the zinc in a solution of copper sulfate instead of pure water. As you will recall if you have seen this commonly-performed experiment carried out, the zinc metal quickly becomes covered with a black coating of finely-divided metallic copper. The reaction is a simple oxidation-reduction process, a transfer of two electrons from the zinc to the copper ... 

Continuous efforts have therefore been made to develop binders for one-pack paints which must be completely compatible with active pigments such as zinc dust. Mixtures of ethyl silicate and zinc can be stored for unlimited periods in the absence of water. If the hydrolysis and condensation reactions caused by atmospheric moisture can be sufficiently accelerated during application of the coating, a one-pack system can be produced. 

Various bolt/nut combinations have been used with modular seals, ranging from vanilla cadmium-plated carbon steel to today s more exotic metric two-part zinc dichromate sub-coatings per ASTM B-663 under a propriety organic outer coating for superior corrosion resistance. To benchmark the corrosion resistance of these different combinations, four bolts were continuously immersed in tap water for four years, the results of which are shown in Figure 5. 

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