Chromium oxide film, protective

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... 

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. 

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. 

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 . 

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... 

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. 

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. 

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

Concentrated nitric acid passivates many metals, such as, iron, cobalt, nickel, aluminum and chromium, forming a protective film of oxides on their surfaces, thus preventing any further reaction. Very dilute nitric acid is reduced by a strong reducing agents, such as metallic zinc, to form ammonia and hydroxylamine, NH2OH. 

At the other extreme, the oxide layers on aluminum, beryllium, titanium, vanadium, chromium, nickel, and tantalum are very insoluble in water at intermediate pH values and do not have easily accessible reduced states with higher solubility. The oxide films on those metals are therefore highly protective against aqueous corrosion. 

The common feature of all stainless steels is the presence of at least 11%, and more usually 18%, chromium. This confers corrosion protection through formation of a particularly insoluble and reduction-resistant oxide film. The steels are heat-treatable for improved mechanical properties (Section 5.7) if they can undergo the a-Fe/y-Fe transition at elevated temperatures. It is useful to distinguish four groups of commonly used stainless steels ... 

Active metals such as aluminum, titanium, and high-chromium steels become corrosion resistant under oxidizing conditions because of a very adherent and impervious surface oxide film that, although one molecule thick, develops on the surface of the metal. This film is stable in a neutral medium, but it dissolves in an acid or alkaline environment. In a few cases, such as certain acid concentrations, metals can be kept passive by applying a carefully controlled potential that favors the formation of the passive surface film. The ability to keep the desired potential over the entire structure is very critical in anodic control. If a higher or lower potential is applied, the metal will corrode at a higher rate, possibly higher than if it is not protected at all. 

Chromates and molybdates protect the steel by passivation and form a chromium oxide-iron oxide in the case of chromate. While the oxide film of chromium may be several monolayers, molybdenum oxide film is of the order of a monolayer.73... 

The success of the Type 300 series stainless steel in molten carbonates is a result of the protective LiCr02 film which forms a compact, tenacious, and self-healing layer. This film forms in about 500 hrs and decreases the corrosion rate to a few mils per year. It has been shown that this film is essentially chromium oxide, with the vacant interstices filled with lithium. Lithium is the only stable ionic species present in the melt which is capable of filling the vacant interstice without expanding the oxide lattice (5). Thus a stable diflFusion barrier is formed which limits further corrosion. 

Stainless steel is corrosion resistant because a protective oxide layer naturally forms on top of the surface in the presence of oxygen and humidity. This protective oxide layer typically has a thickness in the order of nanometers, depending on the present environmental conditions. XPS studies of oxide films formed in air on AISI 316 revealed that not only oxidation of the material takes place, but also chromium and metallic nickel accumulate at the interface between oxide layer and bulk material [1]. The protective film is, of course, not perfect but contains defects like inclusions and grain boundaries. At these defects the film may locally break down and dissolution of the bulk material may start [2]. This kind of corrosion is called pitting corrosion and is estimated to cause a third of all chemical plant failures in the United States [3]. 

---