Galvanized steel surfaces

The assessment for nonalloyed ferrous materials (e.g., mild steel, cast iron) can also be applied generally to hot-dipped galvanized steel. Surface films of corrosion products act favorably in limiting corrosion of the zinc. This strongly retards the development of anodic areas. Surface film formation can also be assessed from the sum of rating numbers [3, 14]. 

Cell formation can easily be detected by measuring potential if coated surfaces with no pores have a more positive potential than uncoated material. Usually this is the case with coated steel in solutions containing oxygen. More negative potentials can only arise with galvanized steel surfaces. Figure 5-4 shows examples of measured cell currents [9,10,16]. 

In the presence of a moisture film, but in the absence of pollutants other than CO2, the effective operative electrochemical reactions taking place on a wet galvanized steel surface are believed to be (12)... 

A separate NO2 exposure experiment was conducted to determine whether the amount of NO2 deposited on the surface during a dry period could be ignored relative to that adsorbed in the presence of dew. It was found that saturation occurs on a dry galvanized steel surface and that the adsorbed NO2 saturation surface density was approximately 2 nmol/cm2. The saturation density is less than 5 of the amount of NO2 deposited per 7 h of dew (41 nmol/cm2), and therefore its contribution can be ignored. 

Fig. 1 Morphology of the phosphate layer field emission microscopy images of a trication phosphated galvanized steel surface (a) after complete formation of the phosphate layer and (b) early stages of phosphating (several seconds...

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.

The effect of the phosphate layer as a cathodic inhibitor under atmospheric conditions is illustrated in Fig. 12. In this experiment, the Volta potential of a galvanized steel surface is measured as a function of time during a transition from air to Ar atmosphere, as indicated [51]. The measurement is performed with a Kelvin probe, and the Volta potential of the corroding surface is directly proportional to the corrosion potential with appropriate calibration [58]. The potential jump induced by the presence of air is a measure of the sensitivity of the surface to the oxygen reduction reaction. Here, we see that the galvanized steel surface shows a very large potential jump, on the order of 200 mV. However, the phosphated surface shows only... 

The conclusion of these experiments was that the carbon deposits served as cathodic sites on the phosphated galvanized steel surfaces. 

Fig. 12 The phosphate coating as a cathodic inhibitor under atmospheric conditions. Volta potential transients of a phosphated galvanized steel surface during a transition from Arto humid air and back to Ar. The sensitivity of the surface to oxygen reduction is indicated by the magnitude of the potential jump (a) Galvanized steel without phosphate (b) with phosphate (c) phosphate layer with carbon impurities. (From Ref [51].)...

It is obvious that in case of a defect down to steel, which leads to the enhanced anodic dissolution of zinc, the delamination of the purely alkaline cleaned galvanized steel surface is not faster than that of a phosphated surface. Such a behavior can be explained by an anodic delamination process. If the corrosion conditions are such that no formation of a cathode is possible in front of the anode, then just the kinetics of zinc dissolution determine the degradation of the polymer-metal composite. 

Figure 10 Element maps of carbon (Cls line, top) and zinc (ZnLLM line, bottom) recorded by imaging XPS at a corroded area on a galvanized steel surface. The area shown is 5x5 mm. (From Ref. 32.)... 

Dunham K (2002) Preparing hot-dip galvanized steel surfaces for painting or powder coating a primer. Met Finish 100(9) 20. doi 10.1016/S0026-0576(02)807-46-2... 

Galvanized steel. Galvanized steel must be cleaned to remove the oil or wax that is applied at the mill to prevent white corrosion. After cleaning, the surfaces are pretreated using chromates and phosphates. Vinyl wash primer pretreatments can also be applied on galvanized steel surfaces having no other pretreatments. 

The corrosion film of basic zinc carbonate is soluble in clean rain. The dissolution depends on the residence time of rain on the galvanized steel surface. 

Spence and Haynie (1990) combine the dry and wet deposition terms derived for the corrosion of zinc surfaces to propose the following linear damage function for predicting the long-term corrosion (p,m) of galvanized steel surfaces ... 

Formation of brown areas on galvanized steel surfaces starts only if the... 

American Society for Testing and Materials. (1986). D 2092-86. Practices for preparation of zinc-coated galvanized steel surfaces for paint. ASTM Annual Book of Standards, Vol. 06.01. ASTM, Philadelphia. 

Figure 55. Positive SSIMS spectra from each side of the metal-paint interface on a hot-dip galvanized steel surface after accelerated cyclic corrosion testing [1871 A) Metal B) Paint...

Almeida, E., Diamantino, T.C, Figueiredo M.O., Sa C. (1998). Oxidizing alternative species to chromium VI in zinc - galvanized steel surface treatment. Part I-A morphological and chemical study. Surface and Coating Technology, Vol. 106, pp. 8-17. ISSN 0257-8972... 

If a galvanized steel surface is investigated by the scanning Kelvinprobe, two situations have to be distinguished The defect may be prepared only up to the zinc surface and then zinc is present at the metal/poly-mer interface as well as inside the defect, or the defect is prepared down to the steel substrate and then an additional galvanic element is set up between zinc at the interface and zinc at the defect, the interface being the sacrificial anode necessary to cathodically protect the bare iron. 

Under normal circumstances, galvanized steel surfaces may safely be in contact with types 304 and 316F stainless steel, most aluminum alloys, chrome steel (> 12% Cr), and tin, provided the area ratio of zinc to metal is 1 1 or lower and oxide layers are present on both aluminum alloys and the two stainless steels. 

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