Passive galvanized steels

In Fig. 39, the delaminated area of an organic coating on alkaline cleaned galvanized steel and the same coating on phosphated and Cr(VI) passivated galvanized steel are compared. Defects were prepared down to steel and down to zinc. The ten weeks corrosion test... 

A similar coating system (two-coat coilcoating system on phosphated and chromate passivated galvanized steel) was exposed to a pure salt spray test (360 h). The... 

Corrosion of the fasteners occurred due to their galvanic interaction with passive stainless steel. Deterioration was rapid because of the unfavorable area ratio formed by the large areas of stainless steel and the small area of the fasteners, which was further reduced by the incomplete plastic covering overexposed fastener surfaces. 

Sulfuric acid. As a 10 to 20% v/v solution, sulfuric acid can be used to clean 300 series SS, as well as other steels and metals, but not galvanized steel or magnesium. The cleaned SS can then be passivated with nitric acid. In practice, sulfuric acid is seldom used, except by specialist cleaning companies, because of its high heat of dilution and terrible burning effect on skin and other tissues. Add acid to water. 

The solution was applied to the surface of cold-rolled steel, dip-galvanized steel sheet, and aluminum strip for degreasing and passivating in a single operation at 40°C for 90 s. The surfaces were then lacquered (85-100 pm thickness) and tested in a salt-spray test for 480 h without showing subsurface migration and blistering. The method is especially useful in the automobile industry for coated sheet [191]. 

Is the cooling tower for HVAC purposes and constructed of galvanized steel Did the initial treatment program encompass a passivation program to prevent the risk of white rust Can the tower cope with peak summer cooling demands ... 

The corrosion film on zinc contains about 65 wt pet ZnC03 and 35 wt pet Zn(0H)2 The corrosion film from the Washington, DC, site may also contain ZnO. The corrosion film on galvanized steel consists of ZnC03, ZnCOH), and ZnO. The presence of the ZnO is probably due to the stabilization of an initial passive ZnO film by Cr which persists over parts of the surface in exposures up to 3 years. 

Presence of different metals. Rebars of carbon steel in certain cases can be connected to rebars or facilities made of stainless steel or copper. This type of coupling, which in other electrolytes would provoke a considerable degree of corrosion in carbon steel by galvanic attack, does not cause problems in the case of concrete any different from those provoked by coupling with normal passive steel. In fact, the corrosion potential of passive carbon steel in concrete is not much different... 

The external layer of pure zinc is of primary importance with regard to the corrosion resistance of the bars. If galvanized steel is exposed to a neutral environment, such as the atmosphere, the duration of protection is primarily dependent on the thickness of the zinc coating, and its composition and microstructure has a negligible effect. Similarly, for galvanized-steel bars embedded in concrete, the protective properties of zinc coatings are due for the most part to the external layer of pure zinc, which can form a passive film if it has a sufficient thickness [36-39]. In fact, a loss of thickness of 5-10 pm is required prior to passivation, while, if the thickness is insufficient, the underlying layers of Zn-Fe alloy passivate with more difficulty. 

The passive film that forms on zinc not only reduces the rate of the anodic process (zinc dissolution), but even hinders cathodic reactions of oxygen reduction and hydrogen development. In conditions of passivity, the corrosion potential of galvanized steel is therefore much lower than that of carbon steel. Values typically measured are between -600 and -500 mV SCE compared to values above -200 mV usually found for passive carbon-steel reinforcement. 

The passive film of galvanized rebars is stable even in mildly acidic environment, so that the zinc coating remains passive even when the concrete is carbonated. The corrosion rate of galvanized steel in carbonated concrete is approximately 0.5-0.8 pm/y, therefore a typical 80 pm galvanized coating would be expected to last over 100 y. [44]. The corrosion rate of galvanized bars remains negligible in carbonated concrete even if a low content of chloride is present. 

Fig. 11 Passivating effect of a phosphate layer. Distribution of vertical component of current over a scratched phosphated galvanized steel surface measured by the SVET method. The scratch penetrates down to the steel surface. Cathodic zones (current < 0) are indicated by the filled areas, while anodic zones (current > 0) are transparent. Because of the passivating properties of the phosphate layer, the anodic reaction remains localized in the vicinity of the scratch defect. Each isocurrent line represents lOpAcm. Original data Irsid.

In Fig. 19(a), a defect in a polymer coating on a metal substrate is shown. The anodic dissolution in the defect leads to a positive current peak. The intact polymer-coated area shows zero current. A second important situation, a cut edge of coil-coated galvanized steel, is shown in Fig. 19(b). The zinc dissolution leads to a positive peak while the area of oxygen reduction on the cathodically protected steel surface is characterized by a broad negative current peak. The activation, distribution, and passivation of these local anodes and cathodes can now be studied by the SVET as a function of coating compositions. 

F. Tittarelh, T. BeUezze, Investigation of the major reduction reaction occurring during the passivation of galvanized steel rebars, Corros. Sci. 52 (2010) 978—983. 

Although zinc can be used in the form of rolled sheet and strip, its widest use in building is for protective plating. When exposed to weather, zinc on the surface of galvanized steel develops a passive film that protects the underlying zinc and, thus, the steel. When the surface is scratched or scored, the anodic zinc corrodes, protecting the cathodic steel from attack. 

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