Passivity condition

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. 

Testing under energized conditions Testing equipment under energized conditions should be performed only after tests under passive conditions have been conducted successfully. The same test in four different positions, under energized conditions, should be conducted and the behaviour and performance of the equipment assessed. 

Sulphuric acid is frequently made, stored and conveyed in lead. The corrosion resistance is excellent (see Figure 4.15) provided that the sulphate film is not broken in non-passivating conditions. Rupture of the film may be caused by erosion by high velocity liquids and gases containing acid spray. 

The potentiostat has supplied an experimental tool for the study of anodic protection. The elucidation of passive behaviour made possible by poten-tiostatic anode polarisation curves allowed investigators to determine the conditions necessary for maintaining a metal in a stable passive condition by provision of a suitable environment, addition of cathodic alloying elementsand/or maintenance of the required potential by means of external anodic polarisation - . ... 

Alloying to modify the overpotential of the metal surface for H2 evolution or O2 absorption can help control corrosion, although it is not always obvious whether these cathodic processes should be suppressed (i0 lowered) or stimulated to produce the desired corrosion resistance. In the case of titanium (see Section 16.6), for example, palladium was alloyed in to catalyze H2 evolution and to force the metal into a passive condition. 

Anodic protection is less commonly used and relies on an external potential control system to maintain the metal in a passive condition. This form of corrosion protection has found practical application in the sulphuric acid manufacturing industry. 

Film is less porous to corrosive ions which largely remain outside film. At the substrate, ratio of inhibitive ions to corrosive ions is conducive to formation and preservation of passive condition. 

In the first group, emerging persistent slip bands (PSBs) are preferentially attacked by dissolution. This preferential attack leads to mechanical instability of the free surface and the generation of new and larger PSBs, followed by localized corrosion attack, resulting in crack initiation. Under passive conditions, the relative rates of periodic rupture and reformation of the passive film control the extent to which corrosion reduces fatigue resistance. When bulk oxide films are present on a surface, rupture of the films by PSBs leads to preferential dissolution of the fresh metal that is produced.102... 

At the anodes, iron is dissolved and an oxide deposited. Electrons travel from anode to cathode within the metal, and OH ions travel from cathode to anode through the solution with which it is in contact. For these processes to occur, a continuing source of oxygen is needed, and the surface of the metal must remain wet. If pH is above about 11.5 (P48) and Cl" is absent, the oxide is deposited as a thin protective film which is virtually continuous, and the rate of attack is so low as to be insignificant. The iron is said to be in a passive condition. At a lower pH, an oxide or oxyhydroxide is deposited in an incoherent form, and corrosion is rapid. 

On the other hand, the situation depicted by curve Y will not allow (ia)j to be exceeded the metal will not become passive but may assume the steady state condition at point J. However If the metal has been previously passivated by an air formed film or other means, it may be maintained in the passive condition shown by point K. 

Figure 3 Quenching results, (/o//)- 1, of 2-, 9-, and 16-AF using KI under passive conditions and iontophoresis at 100 and 300 pA/cm, together with the control involving iontophoresis in Hepes buffer alone at 1000 pA/cm. ...

Metals of the iron group and chromium can be made passive by heating in air, and a semi-passive state, in which the metal exhibits an electrode potential between the values for the completely active and completely passive conditions, can be induced in these metals, as well as in molybdenum, tungsten and vanadium, by mere exposure to air. The corrosion resisting properties of stainless steel and of chromium plate are undoubtedly due to passivity resulting from exposure to air. 

In the passive condition chromium is corrosion-resistant. It is therefore used as a protective electro-plating on ferrous metals and as a component of stainless steels. The metal dissolves slowly in cold dilute HCl and H2SO4, not at all in HNO3, but rapidly in hot HCl and in hot cone. H2SO4. It reacts with CI2 and Brg when heated, with Og at the temperature of the oxy-hydrogen flame, and with H2O at red heat ... 

The Passivity of Metals. It has been known for a long time that a number of metals can exist in two states, in one of which they exhibit greater activity, at least with respect to certain types of chemical reactions, than the other. In the first case the metals are said to be in the active and in the second to be in the passive condition, the two terms being due to Schonbein1 who made some of the early studies of the phenomena. The most familiar case of passivity is that of iron. A piece of iron when placed in dilute nitric acid will normally dissolve with the evolution of hydrogen, and is in its active state. If it is transferred to concentrated nitric acid and returned once more to the dilute acid little or no reaction will take place. It is now in its passive condition. The active state may be regained by touching the surface with a piece of the active metal, and by other means. In the passive state the metal behaves as if it is more noble, L e,3 has a position lower in the electromotive series, than in the active condition. 

Iron unites directly with Cl, Br, JL K, P, As, and Sb. It dioaolvea in HCl as ferrous chloride, while H is liberated. Heated with strong H,SO it gives off SO, with dilute H,SO, H is given off and lenrous sulphate formed. Dilute HNO, dissolves Fe, but the concentrated acid renders it josfticc, when it is uot dissolved by either concentrated or dilute HNO until the passive condition is destroyed by contact with Pt, Ag or Cu, or by heating to 40 (104 F.). 

---