Model Electrochemical Cell Study of Cut-Edge Corrosion Inhibition on Coil-Coated Steel Sheet by Chromate-, Phosphate-, and Calcium-Containing Pigments

Abstract

Corrosion at cut edges is the most important failure mechanism of organic-coated, profiled galvanized steel architectural claddings. Currently, edge corrosion is generally controlled by the addition of strontium chromate in the paint primers; however, there is substantial interest in chromate replacements due to environmental reasons. This work describes an experimental study of inhibition with specific relevance to the cut-edge situation; essentially equivalent to a small galvanic cell between zinc and steel. Although chromate initially acts as an anodic inhibitor for zinc corrosion at the cut edge, over a few hours of immersion, it was found to also strongly inhibit the steel cathode, hence reducing the cathodic protection current requirement on the zinc and thus acted as a mixed inhibitor in the cut-edge galvanic cell. Although individually, zinc phosphate and calcium ion-exchanged silica pigments had relatively poor inhibition, they showed a strong synergistic effect. Thus, a mixture of the two compounds had comparable inhibitive efficiency to chromate. This was found to be due to a similar mixed inhibition mechanism as chromate. Thus, anodic inhibition of zinc was evident as well as strong cathodic inhibition on the steel due to the formation of a compact, thin film containing zinc, calcium, and phosphate species.Combined metal/polymer coatings are widely used in industrially developed countries for protection of different objects from atmospheric corrosion. Their application has a great economic benefit compared with organic and metallic coatings applied separately 1-3 because the durability is dramatically increased. These combined coatings consist of a metallic layer ͑e.g., zinc, aluminum, or their alloys͒ applied by different methods with hot dipping mostly used for this purpose. Further, an organic coating, which includes a primer and top coat, is applied onto the metallic layer after pretreatment. These are then usually profiled before being used for cladding ͑roofing and siding͒ of buildings. Although coil-coated products are used for other applications including domestic appliance casings ͑''white goods''͒ and in the automotive industry, this work focuses on their use in the building industry.As the application of the anticorrosion system is carried out under factory conditions the quality of the protection achieved is very high. However, mechanical damage to the coating may take place either during transportation or construction. In addition, the edge of the steels where they have been cut are clearly uncoated. Thus, the unprotected cut edges corrode under the additional influence of a galvanic couple between the exposed rim and steel. 4,5 Under the circumstances the zinc layer serves as a sacrificial anode and the steel as a cathode. The zinc layer has a typical thickness of 15 m, while the typical thickness of steel is from 0.4 to 1.4 mm. Thus, the current density on the anode is relatively high and accelerated zinc dissolution will take place. One effective means for the control of zinc dissoluti...

Effect of Phosphate and Chromate Pigments on Sacrificial Corrosion Protection by Al–Zn Coating and Delamination Mechanism of Pre-painted Galvalume Steel

“…In phosphate or chromate containing solutions, a deposition layer composed of phosphates or Cr(III) oxyhydroxides was probably formed on steel substrate, inhibiting the oxygen reduction reaction. 4,18,21,26) However, the KPFM results of this study indicate that the corrosion potential of the steel substrates was not significantly influenced by the anti-corrosive pigments; as such, the pigments themselves appear to act as anodic inhibitors that suppress the dissolution rate of the Al-Zn layer.…”

“…Some researchers have focused on the effect of phosphate pigments on the corrosion resistance of Zn-coated steels. [18][19][20][21][22][23] Simões et al studied the corrosion inhibition mechanism of sodium phosphate (Na 3 PO 4 ) at the cut edges of electrogalvanized steels using scanning vibration electrode technique (SVET) and electrochemical impedance spectroscopy (EIS) in phosphate-containing 0.1 M NaCl solution. They summarized that Zn 3 (PO 4 ) 2 , which was formed by the reaction between phosphate and Zn ions, inhibited the anodic dissolution of Zn; 18,19) however, the protective performance of Zn 3 (PO 4 ) 2 was lower than that of zinc chromate.…”

“…20) Zin et al reported that the deposition of compact calcium/zinc phosphate species on the Zn coating layer and the steel substrate highly improved the efficiency of the inhibition effect of phosphates on cut-edge corrosion. [21][22][23] These studies were carried out in immersion conditions in aqueous solutions. As such, the corrosion protection mechanisms of phosphate pigments on pre-painted steels in atmospheric environments remain unclear.…”

Electrochemical Roles of Anti-corrosive Pigments in Sacrificial Corrosion Protection of Painted Galvanized Steel and their Relation to Organic Coating Delamination

“…13) Zin et al assessed the galvanic reactions between Zn and steel using a model cell of cut edges. 14) They demonstrated that chromate initially acts as an anodic inhibitor for Zn corrosion, and after a few hours, it prevents the cathodic reaction on the steel. Muto et al reported that chromate inhibited the oxygen reduction reaction on the steel substrate, providing the conditions for the Zn layer to act as a sacrificial anode.…”

An organic coating pigmented with strontium aluminium polyphosphate for corrosion protection of zinc alloy coated steel

“…The polymer binder and barrier fillers together contribute to the barrier protection of the metal substrate, while active protection is achieved by incorporation of corrosion inhibitive pigments [4][5][6][7]. Phosphate and polyphosphate inhibitive pigments, which are combined with one or more cations of zinc, calcium, aluminium, lithium, magnesium and strontium [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22], are commonly used to replace traditional chromate-containing pigments that are environmentally hazardous and carcinogenic. These phosphate or polyphosphates pigments are assumed to be sparingly soluble in water but release free ions that protect the metal substrate at locations of coating damage by passivating the exposed metal or by otherwise forming a protective, precipitated, salt film.…”