Chromium oxide films

The most important treatment is the conversion coating (see Chapter 5.3 of the same volume). This type of treatment is typically used for zinc or cadmium layers or on bulk metals like aluminum or magnesium. The classical conversion coating is chromating, the formation of a metal oxide/chromium oxide film. We will discuss the process for the example of zinc layers. 

But we were lucky in understanding the biocide used. After a careful examination of the available data, we realized that the biocide used was an SRB-specific one and not a broad-spectrum one. Even worse, the mechanism by which the biocide was acting was via creating chlorides in the environment. These chloride ions, while useful in killing the bacteria, were also detrimental to the stainless steel s protective chromium oxide film. By reacting with this film, the chloride ions dissolve the film, and thus, stainless steel loses its corrosion resistance. 

Stainless steel refers to steel alloys with a minimum chromium content of 10.5% that forms a thin chromium oxide film on the steel s surface to improve corrosion resistance, which makes the steel stainless. Other alloying elements typically include nickel and molybdenum to improve materials properties, e.g. fabrication. 

The chromium oxide film on the surfaces prevents wetting of the base metal by the molten filler and must therefore be removed by a suitable flux. Stainless steels can easily be joined together with other metallic materials or stainless steels of other composition. All conventional brazing processes, such as furnace, torch, induction and resistance brazing, can be employed. The most commonly used process is furnace brazing. 

The white storage stains on zinc and aluminum coatings can be prevented via application of a chromate passivation treatment, which forms a hydrated chromium oxide film on the surface. 

When a chromate-containing paint is exposed to moisture, the chromate ions leach from the coating through to the substrate metal, where they are reduced to form a protective chromium oxide film. The most common inhibiting pigments used in primer paints are the slightly soluble salts of zinc, barium and strontium they provide leachable chromate at a slow, controlled rate. 

Figure 6.13 Formation of a mixed iron oxide and chromium oxide film...

Chromate(VI)-based pre-treatments have historically been used to protect aluminium alloys. When the Cr(VI) species comes into contact with an aluminium substrate, a complex chromium oxide film is formed which provides excellent corrosion protection. Unfortunately, due to their toxicity and adverse environmental impact, these types of pre-treatment will be banned within Europe and North America. Many reports have shown that chromium(VI) is a carcinogen, and can cause kidney and liver damage, and even death [2,3]. Henee, there is an urgent need to provide corrosion protection systems for aluminium, and other active systems, that are accepted as being environmentally-compliant . 

Copper and Copper-Containing Alloys. Either sulfuric or hydrochloric acid may be used effectively to remove the oxide film on copper (qv) or copper-containing alloys. Mixtures of chromic and sulfuric acids not only remove oxides, but also brighten the metal surface. However, health and safety issues related to chromium(VT) make chromic acid less than desirable. 

Hard plating is noted for its excellent hardness, wear resistance, and low coefficient of friction. Decorative plating retains its brilliance because air exposure immediately forms a thin, invisible protective oxide film. The chromium is not appHed directiy to the surface of the base metal but rather over a nickel (see Nickel and nickel alloys) plate, which in turn is laid over a copper (qv) plate. Because the chromium plate is not free of cracks, pores, and similar imperfections, the intermediate nickel layer must provide the basic protection. Indeed, optimum performance is obtained when a controlled but high density (40—80 microcrack intersections per linear millimeter) of microcracks is achieved in the chromium lea ding to reduced local galvanic current density at the imperfections and increased cathode polarization. A duplex nickel layer containing small amounts of sulfur is generally used. In addition to... 

Chromium is an essential constituent in alloys to be used above 550°C (1,000°F). It provides a tightly adherent oxide film that materially retards the oxidation process. Sihcon is a usebil element in imparting oxidation resistance to steel. It will enhance the beneficial effects of chromium. Also, for a given level of chromium, experience has shown oxidation resistance to improve as the nickel content increases. 

In the stainless group, nickel greatly improves corrosion resistance over straight chromium stainless. Even so, the chromium-nickel steels, particularly the 18-8 alloys, perform best under oxidizing conditions, since resistance depends on an oxide film on the surface of the alloy. Reducing conditions and chloride ions destroy this film and bring on rapid attack. Chloride ions tend to cause pitting and crevice... 

Stainless steels contain 11% or more chromium. Table 5.1 lists common commercial grades and compositions of stainless steels. It is chromium that imparts the stainless character to steel. Oxygen combines with chromium and iron to form a highly adherent and protective oxide film. If the film is ruptured in certain oxidizing environments, it rapidly heals with no substantial corrosion. This film does not readily form until at least 11% chromium is dissolved in the alloy. Below 11% chromium, corrosion resistance to oxygenated water is almost the same as in unalloyed iron. 

The triggering mechanism for the corrosion process was localized depassivation of the weld-metal surface. Depassivation (loss of the thin film of chromium oxides that protect stainless steels) can be caused by deposits or by microbial masses that cover the surface (see Chap. 4, Underdeposit Corrosion and Chap. 6, Biologically Influenced Corrosion ). Once depassivation occurred, the critical features in this case were the continuity, size, and orientation of the noble phase. The massive, uninterrupted network of the second phase (Figs. 15.2 and 15.21), coupled... 

A considerable quantity of this foreign element is needed to give adequate protection. The best is chromium, 18% of which gives a very protective oxide film it cuts down the rate of attack at 900°C, for instance, by more than 100 times. 

The extent of the corrosion depends on the amount of nickel and chromium in the alloy. The oxide films become porous and nonprotective, which increases the oxidation rate (accelerated oxidation). 

In XPS, chemical information is comparatively slowly acquired in a stepwise fashion along with the depth, with alternate cycles of sputtering and analysis. Examples of profiles through oxide films on pure iron and on Fe-12Cr-lMo alloy are shown in Fig. 2.9, in which the respective contributions from the metallic and oxide components of the iron and chromium spectra have been quantified [2.10]. In these examples the oxide films were only -5 nm thick on iron and -3 nm thick on the alloy. 

The high-chromium irons undoubtedly owe their corrosion-resistant properties to the development on the surface of the alloys of an impervious and highly tenacious film, probably consisting of a complex mixture of chromium and iron oxides. Since the chromium oxide will be derived from the chromium present in the matrix and not from that combined with the carbide, it follows that a stainless iron will be produced only when an adequate excess (probably not less than 12% of chromium over the amount required to form carbides is present. It is commonly held, and with some theoretical backing, that carbon combines with ten times its own weight of chromium to produce carbides. It has been said that an increase in the silicon content increases the corrosion resistance of the iron this result is probably achieved because the silicon refines the carbides and so aids the development of a more continuous oxide film over the metal surface. It seems likely that the addition of molybdenum has a similar effect, although it is possible that the molybdenum displaces some chromium from combination with the carbon and therefore increases the chromium content of the ferrite. 

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