Passivity in sulfuric acid

Molybdenum in combination with chromium increases the corrosion-resistant properties of ferritic stainless steel in chloride electrolytes and is effective in increasing the resistance to pitting and crevice corrosion. Cr-Ni-Mo-Cu alloys increase the passivity in sulfuric acid concentrations with concentrations between 20% and 70%. Nickel... 

M.P. Ryan, R.C. Newman, G.E. Thompson, Atomically resolved STM of oxide film structures on Fe-Cr alloys during passivation in sulfuric acid solution, J. Electrochem. Soc. 141 (1994) L164-L165. 

K. Tachibana, K. Miya, K. Furuya, G. Okamoto, Changes in the power spectral density of noise current on type 304 stainless steels during the long time passivation in sulfuric acid solutions, Corros. Sci. 31 (1990) 527-532. 

Additions of chromium to nickel impart resistance to oxidizing conditions (e.g., HNO3 and H2Cr04) by supporting the passivation process. The critical minimum chromium content [4] obtained from critical current densities for anodic passivation in sulfuric acid is 14 wt.% Cr. These alloys are more sensitive than nickel to attack by CT and by HCl, and deep pits form when the alloys are... 

M. Prazak constructed an original electronic potentiostat which became the basic instrument for study of corrosion. It was used, for example, for following corrosion of corrosion-proof steels and alloys [122-124]. The effects of temperature on potentials of metals, on kinetics of their dissolution, and on activation energy of iron passivation in sulfuric acid were tested in the institute [125-130]. 

Copper-containing lead alloys undergo less corrosion in sulfuric acid or sulfate solutions than pure lead or other lead alloys. The uniformly dispersed copper particles give rise to local cells in which lead forms the anode and copper forms the cathode. Through this anodic corrosion of the lead, an insoluble film of lead sulfate forms on the surface of the lead, passivating it and preventing further corrosion. The film, if damaged, rapidly reforms. 

Anodic Protection This electrochemical method relies on an external potential control system (potentiostat) to maintain the metal or alloy in a noncorroding (passive) condition. Practical applications include acid coolers in sulfuric acid plants and storage tanks for sulfuric acid. 

It is known that the common austenitic stainless steels have sufficient corrosion resistance in sulfuric acid of lower concentrations (<20%) and higher concentrations (>70%) below a critical temperature. If with higher concentrations of sulfuric acid (>90%) a temperature of 70°C is exceeded, depending on their composition, austenitic stainless steels can exhibit more or less pronounced corrosion phenomena in which the steels can fluctuate between the active and passive state [19]. 

Anodic protection is particularly suitable for stainless steels in acids. Protection potential ranges are given in Section 2.4. Besides sulfuric acid, other media such as phosphoric acid can be considered [13,21-24]. These materials are usually stable-passive in nitric acid. On the other hand, they are not passivatable in hydrochloric acid. Titanium is also a suitable material for anodic protection due to its good passivatability. 

Anodic passivation can be observed easily and clearly with iron group metals and alloys as shown in Fig. 11-10. In principal, anodic passivation occurs with most metals. For instance, even with noble metals such as platinum, which is resistant to anodic dissolution in sulfuric acid solutions, a bare metal surface is realized in the active state and a superficial thin oxide film is formed in the passive state. For less noble metals of which the affinity for the oxide formation is high, the active state is not observed because the metal surface is alwa covered with an oxide film. 

Grauel and Krischer also detected similar stationary domains during the oxidation of H2 on a Pt ring electrode in sulfuric acid [182]. As can be seen in the cyclic voltammogram in Fig. 46a, in this case the homogeneous active branch coexisted with the oxide covered passive branch, and thus the homogeneous dynamics were bistable. The stationary structure displayed in Fig. 46b spontaneously formed when a... 

The corrosion potentials are in keeping with the experimental observations steels attain passivity in nitric acid and ferric sulfate sulfuric acid solutions and undergo corrosion in 5% sulfuric acid solutions. 

The Figure is a schematic polarization curve for a metal exhibiting typical thin-film active-passive behavior (e.g., Ni or Cr in sulfuric acid). Note that this diagram is for a single redox system, namely M/M+ (i.e.,... 

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-... 

Silicon irons are very resistant to oxidizing and reducing environments, and resistance depends on the formation of a passive film. These irons are widely used in sulfuric acid service, since they are unaffected by sulfuric at all strengths, even up to the boihng point. 

Mechanical Passivity.—In certain instances the dissolution of an anode is prevented by a visible film, e.g., lead dioxide on a lead anode in dilute sulfuric acid this phenomenon has been called mechanical passivity, but it is probably not fundamentally different from the forms of passivity already discussed. The film is usually not completely impervious, but merely has the effect of decreasing the exposed surface of the electrode to a considerable extent the effective c.d. is thus increased until another process in which the metal is involved can occur. At a lead anode in sulfuric acid, for example, the lead first dissolves to form plumbous ions which unite with the sulfate ions in the solution to form a porous layer of insoluble lead sulfate. The effective c.d. is increased so much that the potential rises until another process, viz., the formation of plumbic ions, occurs. If the acid is sufficiently concentrated these ions pass into solution, but in more dilute acid media lead dioxide is precipitated and tends partially to close up the pores the layer of dioxide is somewhat porous and so it increases in thickness until it becomes visible. Such an oxide is not completely protective and attack of the anode continues to some extent it is, however, a good conductor and so hydroxyl ions are discharged at its outer surface, and oxygen is evolved, in spite of its thickness. 

Fig. 17M Schematic representation of the corrosion and passivation of iron in sulfuric acid. The primary passivation potential and the corresponding critical current density for corrosion i are shown. Breakdown of the passive film occurs at potentials more positive than E. ...

C. Chemical modification of the glued surfaces by the formation of passivating layers. The modification technique depends on the nature of the metal. The parts are most often subjected to acid pickling, e.g. aluminum alloys are anodized in sulfuric and chromic acids. It is preferable to anodize aluminum parts in sulfuric acid followed by treatment of the anodic film in a bichromate. There are several methods of pickling carbon and stainless steels, chemical oxidation of magnesium alloys as well as copper and titanium alloys before gluing [4]. 

De Gromoboy and Shreir(43) argued in their experimental study of nickel in sulfuric acid that higher oxides may form directly from the metal and at the metal surface the observed "passivation potentials" (determined from anodic charging curves) were found to correspond closely with the potentials calculated for Ni - NiO, Ni Ni O, Ni - Ni.,0, and... 

---