Passivation chromium surface

B.E. Wilde and F.G. Hodge, The Cathodic Discharge of Hydrogen on Active and Passive Chromium Surfaces in Dilute Sulphuric Acid Solutions, Electrochim. Acta, Vol 14, 1969, p 619-627... 

Components are dipped in chromate solutions which clean and passivate the surface to some extent by formation of a film of Mg(OH)2 and a chromium compound. Such films have only a slight protective value, but form a good base for subsequent organic coatings (Loose, 1946 Makar and Kruger, 1993). On high-purity alloys, the stronger acid chromates can provide the best paint base (Hillis and Murray, 1987). 

The passive film formed on austenitic stainless steel is duplex in nature, consisting of an inner barrier oxide film and an outer deposit of hydroxide or salt film. Passivation takes place by the rapid formation of surface-absorbed hydrated complexes of metals that are sufficiently stable on the alloy surface that further reaction with water enables the formation of a hydroxide phase that rapidly deprotonates to form an insoluble surface oxide film. The three most commonly used austenite stabilizers—nickel, manganese, and nitrogen—all contribute to the passivity. Chromium, a major alloying ingredient, is in itself very corrosion resistant and is foimd in greater abundance in the passive film than iron, which is the major element in the alloy. 

In general, the higher the oxidation potential the lesser the tendency to corrode. However, some metals corrode less than other metal with higher redox potential. For example, chromium (—0,74 V), zinc (—0,76 V), titanium (—0,89 V), aluminum (—1,71 V) etc. withstand corrosion much better than iron (—0,42 V). This is due to the fact that the surface of these metals coats with an insoluble very thin layer, just a veil, of hard-bitten oxide not reactive at all that, at variance with rust, passivizes the surface blocking the prosecution of corrosion. Table 13.2 provides a synoptic picture of the standard potentials, the so called electrode potential, relative to oxidation reactions of various metals. The standard electrode potential, abbreviated as , is given in volts and is the measure of the potential of any individual metal electrode which is with solute at an elfective concentration of 1 mol/dm at 1 atm of pressure. These potentials are referred to a hydrogen electrode whose reference potential is assumed equal to zero. This is because it is not possible to measure experimentally the value of the dilference of potential Ay between an electrode and its solution as, for example, in the case of zinc reaction (13.16), because any device used for making the measurement must be inserted in the circuit with two electrodes of which one is put in contact with the metal electrode of interest and the other with the solution. Now, this second electrode creates necessarily another interface metal-solution and the potential difference provided by the system is that between the two metals, without any possibility to infer the absolute value of each of them. This is why it is necessary to introduce a reference electrode, which any other potential can be referred to. To... 

Stainless steel develops a passive protective layer (<5-nm thick) of chromium oxide [1118-57-3] which must be maintained or permitted to rebuild after it is removed by product flow or cleaning. The passive layer may be removed by electric current flow across the surface as a result of dissinulat metals being in contact. The creation of an electrolytic cell with subsequent current flow and corrosion has to be avoided in constmction. Corrosion may occur in welds, between dissimilar materials, at points under stress, and in places where the passive layer is removed it may be caused by food material, residues, cleaning solutions, and bmshes on material surfaces (see CORROSION AND CORROSION CONTROL). 

The low current efficiency of this process results from the evolution of hydrogen at the cathode. This occurs because the hydrogen deposition overvoltage on chromium is significantly more positive than that at which chromous ion deposition would be expected to commence. Hydrogen evolution at the cathode surface also increases the pH of the catholyte beyond 4, which may result in the precipitation of Cr(OH)2 and Cr(OH)2, causing a partial passivation of the cathode and a reduction in current efficiency. The latter is also inherently low, as six electrons are required to reduce hexavalent ions to chromium metal. 

Cathodic additions (such as copper and chromium) to low-alloy steels influence the rate of rusting by raising the potential of the surface to more noble values so encouraging passivation Electrochemical measurements certainly seem to bear this out and they have been used in attempts to develop improved compositions . ... 

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