Zinc interface with oxygen

The function of zinc ions may be either catalytic or stractural. Enzymes with a co-catalytic center of two or even three zinc ions in close proximity are also known. In a new type of zinc-binding site, the protein interface, zinc ions are fixed at the interface of two proteins with the aid of amino acid residues. The ligand residues are usually His, Asp, Glu or Cys, which interact via nitrogen, oxygen or sulfur donors with the metal ion. In catalytic binding sites. His coordination dominates and an additional reactive water molecule is bound. 

Oxygen reduction takes place in the defect with a rate that is controlled by the transport of oxygen through the electrolyte layer (a, i). Thus, a galvanic current is established between the anodic site (zinc within the delaminated zone) and the defect (cathode). In the area between the two potential steps (b, ii), no equilibrium potential surface is observed but the potential rises continuously from the borderline of the local anode to the potential jump, which indicates the intact metal-polymer interface. It can be assumed that the closer the zinc to the cathodic delamination front the smaller is the local anodic current while the... 

Additional corrosion may occur where the supply of oxygen at interfaces between joined components is limited. This is not bimetallic corrosion, but its effect at bimetallic joints can be as detrimental as true bimetallic corrosion. However, no crevice corrosion occurred in couples of zinc-aluminum alloys and polyethylene in 10-year atmosphere tests (Noranda, personal communication) nor around nylon bolt heads. Jointing compounds are useful in preventing crevice corrosion as well as bimetallic corrosion. Also, stressed parts of the surface tend to be anodic to unstressed parts, but this effect is not usually of practical significance with zinc and zinc-coated steel. 

Consider a cathodic site where oxygen is diffusing to the metal/electrolyte interface. If an inhibitor, like zinc and magnesium, is added to the metal/electrolyte system, it would react with the hydroxyl ion and precipitate insoluble compounds which would, in turn, stifle the cathodic sites on the metal. In oxygen-induced corrosion. 

Various etchants have been recommended for the remaining metals of engineering practice, but it is doubtful if sufficient work has been reported to differentiate between them or to assess their effect on the durability of the bonds formed with different adhesives. Strong, durable bonds are uncertain with copper because of the ease with which a weak, friable oxide is formed. Even when coated with an adhesive, oxygen can diffuse to the interface and eventually cause failure. Brass has an oxide film almost entirely of zinc oxide and, as with zinc galvanized iron, it can hydrate or form salts with the tackifiers added to some contact adhesives. Cadmium is met with as a plating if a strong, durable adhesive bond is essential, it should be replaced by chromium, the surface of which can be treated as stainless steel. 

The potential difference between the achvely corroding defect and the intact polymer/ metal interface is 500mV for the just oxide-covered zinc siuface and only 150mV for the conversion-coated surface. The delaminahon rate of the transparent polymer film in the con-version-coated area observed to be about 40 times smaller than for the just alkaline cleaned sample. This fact is in qualitative agreement with the measured inhibition of the oxygen reduction. 

---