ANODIC PROTECTION

13.9 ANODIC PROTECTION

As mentioned in Section 6.4 , some metals, such as iron and stainless steels, can also be protected by making them anodic and shifting their potential into the passive region of the anodic polarization curve (see Fig. 6.1 , Section 6.2 ). The passive potential is automatically maintained, usually electronically, by an instru- ment called the potentiostat . Practical application of anodic protection and use

of the potentiostat for this purpose were fi rst suggested by Edeleanu [21] . Anodic protection has found application in handling sulfuric acid [22] , but the method is also applicable to other acids (e.g., phosphoric acid) and to alkalies and some salt solutions. It has been shown to be effective for increasing the

resistance to corrosion fatigue of various stainless steels in 0.5 M Na 2 SO 4 [23] , in 10% H 2 SO 4 or 10% NH 4 NO 3 , and of 0.19% C steel in 10% NH 4 NO 3 [24] . Engineering installations have been described for anodically protecting mild

C ATHODIC PROTEC TION

steel against uniform corrosion in NH 4 NO 3 fertilizer mixtures [25] , carbon steel

in 86% spent sulfuric acid at temperatures up to 60 ° C (140 ° F) [26] , and carbon steel in 0.1 – 0.7 M oxalic acid at temperatures up to 50 ° C (120 ° F) [27] .

Since passivity of iron and the stainless steels is destroyed by halide ions, anodic protection of these metals is not possible in hydrochloric acid or in acid chloride solutions for which the current density in the otherwise passive region

is very high. Also, if Cl − should contaminate the electrolyte, the possible danger

of pitting becomes a consideration even if the passive current density remains acceptably low. In the latter case, however, it is only necessary to operate in the potential range below the critical pitting potential for the mixed electrolyte. * Titanium, which has a very noble critical pitting potential over a wide range of

Cl − concentration and temperature, is passive in the presence of Cl − (low i passive )

and can be anodically protected without danger of pitting even in solutions of hydrochloric acid.

Anodic protection is applicable only to metals and alloys (mostly transition metals) which are readily passivated when anodically polarized and for which passive i is very low. It is not applicable, for example, to zinc, magnesium, cadmium, silver, copper, or copper - base alloys. Anodic protection of aluminum exposed to high - temperature water has been shown to be feasible (see Section 21.1.2 ).

Current densities to initiate passivity, i critical , are relatively high, with 6 A/m 2 being required for Type 316 stainless steel in 67% H 2 SO 4 at 24 ° C (75 ° F). But

currents for maintaining passivity are usually low, with the orders of magnitude being 10 −3 2 A/m (0.1

μ A/cm 2 ) for Type 316 stainless steel to 0.15 A/m 2 (15

μ A/cm 2 )

for mild steel [22] , both in 67% H 2 SO 4 . Corrosion rates corresponding to these

current densities are 0.02 – 2.5 gmd. It is typical of anodic protection that corrosion rates, although small, are not reduced to zero, contrary to the situation for cathodic protection of steel. On the other hand, the required current densities in corrosive acids are usually much lower than those for cathodic protection, since for cathodic protection the current cannot be less than the normal equivalent corrosion current in the same environ- ment. For stainless steels, this value of current density corresponds to the rather high corrosion rate for the active state of the alloys.

For anodic protection, it has been reported [22] that unusual throwing power (protection at distances remote from the cathode or in electrically screened areas) is obtained, far exceeding similar throwing power obtained in cathodic protection. The cause has been ascribed to high electrical resistance of the passive fi lm, but this is probably not correct, because measurements have shown that such resistances are typically low. The cause instead may be related to the corrosion - inhibiting properties of anodic corrosion products released by stainless

steels in small amounts (e.g., SO 2 − , Cr O 2 2 − 8 2 7 , Fe 3+ ), which shift the corrosion poten-

tial into the passive region in the absence of an applied current.

* Stress - corrosion cracking of Type 304 stainless steel, which is reported to occur at room temperature in 10 N H 2 SO 4 + 0.5 N NaCl, is prevented by anodically polarizing the alloy to 0.7 V (S.H.E.). See S. Acello and N. Greene, Corrosion 18 , 286t (1962); J. Harston and J. Scully, Corrosion 25 , 493 (1969).

GENER AL REFERENCES