Iron passive films stainless steel

Monel and nickel are the preferred materials of constmction for cylinders and deHvery systems however, copper, brass, steel, and stainless steel can be used at room temperature, providing that these metals are cleaned, dried, and passivated with a fluoride film prior to use. Studies have shown that fluorine passivation of stainless steel and subsequent formation of an iron fluoride layer prior to WF exposure prevents reaction between the WF and the stainless steel surface (23). 

The passivity of stainless steel is the result of the presence of a corrosion-resistant oxide film on the surface. In most material environments, it will remain in the passive state and tend to be cathodic to ordinary iron or steel. When chloride concentrations are high, such as in seawater or in reducing... 

Molybdenum is an alloying element which is known to increase passivation of stainless steels. Steels of type 316 contain molybdenum as a minor constituent and promote passivation. Sodium molybdate forms a complex passivation film at the iron anode of ferrous-ferric molybdenum oxide. 

Evidence in support of several of these models has been reported. XPS studies of the passive and transpassive films formed on Mo in deaerated 0.1 M HCl [3] established that molybdate was absent from both surface films. In a later study the same authors used a twin potentiostat arrangement, with a second working electrode of either Fe, Cr, or Ni that was polarized near an Mo electrode at the same potential (-180mV vs. SCE) [18]. At this potential Mo and Cr are passive, while Ni and Fe are active. In this work it was shown that for the Fe-Mo and Ni-Mo electrode couples, iron or nickel molybdate was observed on the passive Mo surface. In the case of the Cr-Mo couple, molybdate was observed only on the passive film of Cr. This work was also able to show that transpassivity of Mo at 250 mV (SCE) was suppressed in the presence of Fe, which formed a molybdate salt on the surface of Mo. This indicated evidence of a possible mechanism by which Mo can remain passive in stainless steels at higher potentials than the transpassive potential of Mo. In addition, this work supported the idea that soluble molybdate anions can redeposit at active sites. 

The current-potential relationship ABCDE, as obtained potentiosta-tically, has allowed a study of the passive phenomena in greater detail and the operational definition of the passive state with greater preciseness. Bonhoeffer, Vetter and many others have made extensive potentiostatic studies of iron which indicate that the metal has a thin film, composed of one or more oxides of iron, on its surface when in the passive state . Similar studies have been made with stainless steel, nickel, chromium and other metals... 

Jin and Atrens (1987) have elucidated the structure of the passive film formed on stainless steels during immersion in 0.1 M NaCl solution for various immersion times, employing XPS and ion etching techniques. The measured spectra consist of composite peaks produced by electrons of slightly different energy if the element is in several different chemical states. Peak deconvolution (which is a non-trivial problem) has to be conducted, and these authors used a manual procedure based on the actual individual peaks shapes and peak positions as recorded by Wagner et al. (1978). The procedure is illustrated in Figure 2.8 for iron. 

XPS was also used for the determination of chlorine in the passive film grown in chlorine containing electrolytes. While chlorine was found in the passive film on pure iron, it was absent for chromium rich stainless steel samples. Chloride content of the passive film is substantially time dependent, increasing with time until film breakdown occurs, and decreasing subsequently [109]. 

The passive range of typical stainless steels conveniently spans most of the Eh stability field of neutral water. This can be appreciated by examination of the E° or Eh values for reactions 16.20 to 16.25, with the caveat that these refer to pure iron and chromium metals rather than to stainless steels and that the conditions are standard ones rather than, for example, the very low [Cr3+] in equilibrium with the FeCr2C>4 film. The formation... 

A diagram similar to Fig. 16.8 could be constructed for ordinary iron or steel, but the onset of passivity, corresponding to the formation of an Fe203 film directly over the entire surface of the metal, would occur at a much higher Eh value than for stainless steel [E° for Fe3+(aq)/Fe2+(aq) is +0.77 Vj. The transpassive region, corresponding to anodic oxidation of the... 

Modification of the metal itself, by alloying for corrosion resistance, or substitution of a more corrosion-resistant metal, is often worth the increased capital cost. Titanium has excellent corrosion resistance, even when not alloyed, because of its tough natural oxide film, but it is presently rather expensive for routine use (e.g., in chemical process equipment), unless the increased capital cost is a secondary consideration. Iron is almost twice as dense as titanium, which may influence the choice of metal on structural grounds, but it can be alloyed with 11% or more chromium for corrosion resistance (stainless steels, Section 16.8) or, for resistance to acid attack, with an element such as silicon or molybdenum that will give a film of an acidic oxide (SiC>2 and M0O3, the anhydrides of silicic and molybdic acids) on the metal surface. Silicon, however, tends to make steel brittle. Nevertheless, the proprietary alloys Duriron (14.5% Si, 0.95% C) and Durichlor (14.5% Si, 3% Mo) are very serviceable for chemical engineering operations involving acids. Molybdenum also confers special acid and chloride resistant properties on type 316 stainless steel. Metals that rely on oxide films for corrosion resistance should, of course, be used only in Eh conditions under which passivity can be maintained. 

The stainless steels owe their corrosion resistance to the chromium in the iron alloy, which forms a very thin transparent film of chromium oxide on interaction with oxygen. A ready supply of oxygen with a minimum of 11% of chromium dissolved in the matrix material are necessary conditions to maintain a passive film which is self-healing in air at room temperature. The damage to the passive film results in corrosion in the environment to which the alloy is exposed. The observation that chromium in iron alloy is responsible for corrosion resistance in acid solutions led to the development of Type 430 (16-18% Cr) and Type 446 (24-27% Cr). 

Rust of iron (the most abundant corrosion product), and white rust of zinc are examples of nonprotective oxides. Aluminum and magnesium oxides are more protective than iron and zinc oxides. Patina on copper is protective in certain atmospheres. Stainless steels are passivated and protected, especially in chloride-free aqueous environments due to a very thin passive film of Cr2C>3 on the surface of the steel. Most films having low porosities can control the corrosion rate by diffusion of reactants through the him. In certain cases of uniform general corrosion of metals in acids (e.g., aluminum in hydrochloric acid or iron in reducible acids or alkalis), a thin him of oxide is present on the metal surface. These reactions cannot be considered hlm-free although the him is not a rate-determining one.1... 

Dissimilar metals. Galvanic corrosion occurs when two metals with different electrochemical potentials are in contact in the same solution [Figures 6.7 and 6.8]. In both cases, the corrosion of iron (steel) is exothermic and the cathodic reaction is controlling the corrosion rate. The more noble metal, copper increases the corrosion through cathodic reaction of hydrogen ion reduction and hydrogen evolution A passive oxide film on stainless steel for example can accelerate hydrogen reduction reaction. 

Many metals are resistant to corrosion due to a compact adherent oxide film which acts as a barrier between the metal and its environment. Aluminium and stainless steels are examples of such metals. Often, the properties of such a barrier film will depend on the environment in which the metal is situated. For example iron in seawater does not produce corrosion products which are protective but in dilute carbonate solutions with no or very low chloride concentrations, a passive complex Fe(0H)2/FeC03 layer is formed [19, 20]. 

Bonhoeffer, Vetter, and others (63) have made extensive studies on iron which indicate that the passive film is composed of one or more oxides of iron. Young (64). Vermilyea (65) and Johansen et at, (57) have shown that the Mott-Cabrera concepts are applicable for the thin films on Ta, Ti, Hf, and Hb. Petrocelli (58) has shown evidence that the dissolution of aluminum In sulfuric acid takes place through a thin film and that the process appears to follow the Motr-Cabret a theory. Stern (66) reports data indicating that the kinetic. for the anodic oxidation of stainless steel are similar to those for aluminum apd tantalum (67). Pryor (68) has recently reviewed the work on passive films on iron and suggests a single passive film of y contains non-uniform defect concentra-... 

Passive films formed in aqueous solutions consist of an oxide or a mixture of oxides, usually in hydrated form. The oxide formed on some metals (e.g., Al, Ti, Ta, Nb) is an electronic insulator, while on other metals the passivating oxide film behaves like a semiconductor. Nickel, chromium, and their alloys with iron (notably the various kinds of stainless steel) can be readily passivated and, in fact, tend to be spontaneously passivated upon contact with water or moist air. It should be noted that passivation does not occur when chloride ions are introduced into the solution indeed a preexisting passive film may be destroyed. Many other ions are detrimental to passivity, such as Br, I, SO, and CIO, but chloride is the worst offender, because of its... 

Iron does not passivate in most environments and, therefore, performs best when the oxidizing power of the environment is as low as possible, for example, by deaeration as mentioned above. In contrast, a large class of industrially important alloys depend upon sufficiently oxidizing conditions to produce a protective passive film if they are to perform satisfactorily. These alloys include stainless steels, nickel-base alloys, titanium and its alloys, and many others. 

Fig. 5.24. A similar influence of chromium in nickel is shown in Fig. 5.27. As increasing amounts of chromium are added to iron, the relative fraction present in the passive film increases until, at chromium contents of the large-volume commercial stainless steels (18 to 22% Cr),...

Several experimental results support the adsorption mechanism for stationary conditions of the passive layer. Even the stationary passive current density depends on the composition of the electrolyte. For iron in 0.5 M H2SO4, the passive current density is 7 pA cm , whereas less than lpAcm is detected in 1 M HCIO4. From these observations, a catalysis for the transfer of Fe + from the passive layer to the electrolyte by S04 ions was concluded [55, 56]. Similarly, the dissolution Ni + from passive nickel and nickel base alloys is accelerated by organic acids hke formic acid and leads to a removal of NiO from the passive layer [57]. Additions of citrate to the electrolyte cause the thinning of passive layers on stainless steel and increase its Cr content [58]. Apparently Fe and Ni ions are complexed at the surface of the passive film, which causes an enhancement of their dissolution into the electrolyte. It should be mentioned that the dissolution of Cr " " apparently is not catalyzed by these anions and remains... 

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