that the initial attack on zinc by the alkalies released during hydration 

 of the cement is not progressive and that the coating can be expected to 

 have good durability. Concrete alkalinity is initially corrosive to the 

 zinc coating, forming a layer of zinc hydroxide, and subsequently a complex 

 calcium zincate which is insoluble in the highly alkaline pore liquid in 

 the concrete. The chemical reaction produces a tight bond between the 

 concrete and the zinc-poated steel and provides a barrier against further 

 alkali attack on the underlying zinc. However, the reaction between zinc 

 and the alkaline pore liquid of freshly placed concrete can form bubbles of 

 hydrogen gas which would have an adverse effect on the bond strength in 

 normal reinforced concrete. It has been observed that a small amount of 

 chromate in the cement, or dipping the galvanized steel in a chromate bath, 

 suppresses the evolution of hydrogen. The concentration of chromates in 

 the pore liquid that is necessary to inhibit the formation of hydrogen is 

 very low, on the order of 70 ppm in the cement paste, corresponding to a 

 soluble chromate content of 0.014 percent (by weight) in the dry cement, 

 assuming a W/C of 0.5. Since some Portland cements are low in chromate, 

 chromate-coated galvanized steel should be used with all cements. 



Baker, et al,l^ reported results of an 11-yr study of bare steel and 

 nickel- and zinc-coated steel rebars embedded in concrete castings exposed 

 in the seawater tidal zone. Steel rebar materials were ASTM A-615 steel, 

 a high-strength, low-alloy Ni-Cu-Cr steel, and AISI 4340 steel. The con- 

 crete castings were formulated vrlth ASTM Types I and II Portland cement. 

 The specimens were exposed in the seawater tidal zone, which allowed them 

 to be alternately wet and dry twice each day, and simulated the exposure 

 of reinforced pilings or structures in seawater. The results show that 

 metallic coatings on rebar are definitely beneficial compared to bare carbon 

 or bare low-alloy steels. They also show that a 1-mil nickel coating is 

 sufficient to achieve improved reinforced concrete performance. That study 

 did not reveal any particular advantage in the use of a low-alloy steel over 

 carbon-steel-reinforcing bars. 



Rebar corrosion can also be prevented by providing an adequate concrete 

 cover. A minimum 3-in, -thick cover for plane and curved surfaces and a 

 4-in. -thick cover at corners has been recommended .-'-■■' Browne^ concluded 

 that, for a very wide range of marine applications, reinforced concrete 



