Chemical Composition, Electrochemical, and Morphological Properties of Iron Phosphate Conversion Coatings

• Published in: Corrosion (Houston, Tex.) Vol. 55; no. 9; pp. 898 - 906
• Main Authors: Warburton, Y.J., Gibbon, D.L., Jackson, K.M., Gate, L.F., Rodnyansky, A., Warburton, P.R.
• Format: Journal Article
• Language: English
• Published: Houston, TX NACE International 01.09.1999; NACE
• Subjects: Analytical methods; Animal morphology; Applied sciences; Chemical composition; Coatings; Conversion coatings; Corrosion; Corrosion prevention; Corrosion resistance; Electrochemistry; Electron microscopy; Exact sciences and technology; Heavy metals; Iron; Iron phosphates; Metals. Metallurgy; Morphology; Phosphate coatings; Phosphates; Photoelectron spectroscopy; Photoelectrons; Pretreatment; Properties; Protective coatings; Scanning electron microscopy; Substrates; X ray photoelectron spectroscopy; Zinc; Zinc coatings; Zinc phosphate; Iron phosphate; Scanning electron microscopy; Conversion coating; Salt corrosion; Experimental study; Corrosion protection; Coatings; Surface treatment; Ellipsometry; Corrosion resistance; Electrochemical properties; Corrosion; Morphology; Photoelectron spectrometry; Chemical composition
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3284047

ABSTRACTIron phosphate conversion coatings are used widely in the pretreatment industry to enhance paint adherence to metal substrates and therefore improve corrosion resistance. However, very limited nonproprietary literature describing the properties of iron phosphate coating is available, as compared to volumes dedicated to zinc phosphate coating. The present study described chemical, electrochemical, and morphological characterizations of iron phosphate coating using x-ray photoelectron spectroscopy (XPS), potentiodynamic scans, and scanning electron microscopy (SEM). For the samples under investigation, the mode of operation of iron phosphate coating was to promote paint adhesion, and the coating itself did not impart significant corrosion protection to the metal substrate. It also was shown that the Fe/P ratio in the phosphate coating ranged from 1:2 to 1:1. When tested in pH 7 buffered phosphate solution, the phosphate coating displayed a passivation region, which also possessed the highest impedance value. The phosphate coating was found to comprise two layers: a dense, adherent layer and a loose, granular top layer. For samples with coating weights of 20 mg/ft2 to 30 mg/ft2 (0.22 g/m2 to 0.32 g/m2), the corresponding coating thickness was ~ 0.1 µm to 0.3 µm.Metals play a major role in manufacturing and construction. Among the vast variety of metals available, steel, owing to its low cost, processability, strength, and mature technology, is used widely. Applications range from such areas as military equipment to household appliances. The iron in steel is not thermodynamically stable in its metallic form. It spontaneously reverts to its oxidized form, expressed in the following half cell reaction:Fe ­ 2e- > Fe2+ , E0 = ­ 0.447V SHE1 (1)where E0 is the standard reduction potential. This oxidation of iron is coupled with the reduction of either protons or oxygen, as depicted in the following standard half cell reactions:2H+ + 2e- > H2+E0 = 0.000V SHE1 (2)O2 + 2H2O + 4e- > 4OH- , E0 = 0.401V SHE1 (3)This corrosion process is electrochemical in nature, and, as a result, steel rusts in the presence of elec-