CRITERIA OF PROTECTION

13.7 CRITERIA OF PROTECTION

The effectiveness of cathodic protection in practice can be established in more than one way, and several criteria have been used in the past to prove whether protection is complete. For example, the observed number of leaks in an old buried pipeline is plotted against time, noting that leaks per year drop to a small number or to zero after cathodic protection is installed. Or the hull of a ship can

be inspected at regular intervals for depth of pits. It is also possible to check effectiveness of protection by short - time tests, including the following measures:

1. Coupon Tests. A weighed metal coupon shaped to conform to the outside surface of a buried pipe is attached by a brazed connecting cable, and both the cable and surface between coupon and pipe are overlaid with coal tar. After exposure to the soil for a period of weeks or months, the weight loss, if any, of the cleaned coupon is a measure of whether cathodic protection of the pipeline is complete.

2. Colorimetric Tests. A section of buried pipeline is cleaned, exposing bare metal. A piece of absorbent paper soaked in potassium ferricyanide solu- tion is brought into contact, and the soil is returned in place. After a rela- tively short time, examination of the paper for the blue color of ferrous ferricyanide indicates that cathodic protection is not complete, whereas absence of blue indicates satisfactory protection.

Both the coupon and the colorimetric tests are qualitative and do not provide information about whether just enough or more than enough current is being supplied.

13.7.1 Potential Measurements

A criterion that indicates degree of protection, including overprotection, is obtained through measuring the potential of the protected structure. This mea- surement is of greatest importance in practice, and it is the criterion generally accepted and used by corrosion engineers. It is based on the fundamental concept that optimum cathodic protection is achieved when the protected structure is

CRITERIA OF PROTEC TION

polarized to the open - circuit anode potential of local - action cells. This potential for steel, as determined empirically, is equal to − 0.85 V versus the copper -

saturated CuSO 4 half - cell, or − 0.53 V (S.H.E.). The theoretical open - circuit anode potential for iron can be calculated assum- ing that the activity of Fe 2+ in equilibrium is determined by the solubility of a covering layer of Fe(OH) 2 , in accord with previous discussions of oxide - fi lm composition on iron exposed to aqueous media (see Section 7.1 ). Using the Nernst equation, we obtain

solubility product φ

where Fe ( 2 + ) =

2 ( OH − )) 2

The value of (OH − ) can be estimated assuming that its concentration at equilib- rium is twice that of (Fe 2+ ), in accord with Fe(OH) 2 → Fe 2+

+ 2(OH − ). * The potential so calculated is − 0.59 V (S.H.E.), equivalent to a potential difference of − 0.91 V versus copper - saturated CuSO 4 , and is in essential agreement with the empirical value. The empirical value for lead, known only approximately [19] , is about − 0.78 V versus copper - saturated CuSO 4 compared to the calculated value for a Pb(OH) 2 fi lm on lead equal to − 0.59 V. In alkaline media, with formation of plumbites, the calculated value comes closer to the empirical value. †

Other calculated values are listed in Table 13.2 . For passive metals, the criterion of protection differs from that just described. Since passive metals corrode uniformly at low rates, but by pitting corrosion at high rates, cathodic protection of metals like aluminum and 18 – 8 stainless steel depends on polarizing them not to the usual thermodynamic anode potential, but only to a value more active than the critical potential at which pitting initiates (see Section 6.6 ). The latter potential lies within the passive range and is less

noble the higher the Cl − concentration; in 3% NaCl the value for aluminum is − 0.45 V (S.H.E.). Hence, iron with a corrosion potential in seawater of about − 0.4 V is not suited as a sacrifi cial anode for cathodically protecting aluminum, unlike zinc, which has a more favorable corrosion potential of about − 0.8 V. For

18 – 8 stainless steel, the critical potential in 3% NaCl is 0.21 V; for nickel it is about

0.23 V. Coupling of the latter metals to a suitable area of either iron or zinc, therefore, can effectively protect them cathodically in seawater against initiation

* This relation holds for a saturated solution of Fe(OH) 2 in water, the natural pH of which is 9.5. This pH is observed at an iron surface between an external pH of 4 – 10, as discussed in Section 7.2.3 . Values of surface pH for other corroding metals are specifi c to the metal; but the relation (2M 2+ ) = (OH − ) also applies, provided that the external solution in contact does not alter the natural pH of the surface metal hydroxide.

A similar situation applies to Cd assumed to be coated with a fi lm of Cd(OH) 2 (solubility product = 2

× 10 − 14 ). The calculated value of φ H − 0.54, which is noble to iron, contrary to an observed galvanic is potential less noble than iron (see Fig. 3.3 , Section 3.8 ). The observed more - active galvanic potential of cadmium is plausibly explained by the known tendency of Cd 2+ to form complex ions that lower

Cd 2+ activity below the value corresponding to saturated Cd(OH) 2 .

C ATHODIC PROTEC TION

T A B L E 13.2. Calculated Minimum Potential φ for Cathodic Protection Metal

φ ° (volts)

Solubility Product,

φ H (volts)

φ H versus Cu − CuSO 4

M(OH) 2 Reference Electrode (volts)

of pitting corrosion. Components of practical structures, such as ships and off- shore oil - drilling platforms, are sometimes designed to take advantage of galvanic couples of this kind.

13.7.2 Doubtful Criteria

Criteria have sometimes been suggested based on empirical rules — for example, polarizing a steel structure 0.3 V more active than the corrosion potential. This criterion is not exact and in many situations leads to under - or overprotection. It has also been suggested that polarization of the structure should proceed to breaks in slope of the voltage versus current plot. Such breaks may, in principle, occur in some environments when the applied current is just equal to or slightly greater than the corrosion current (e.g., oxygen depolarization control); but in other environments, breaks may occur when concentration polarization or IR effects through partially protective surface fi lms become appreciable. As Stern and Geary showed [20] , breaks of this kind in polarization measurements have varied causes and are of doubtful value as criteria of cathodic protection. Con- trary to what is sometimes erroneously supposed, discontinuities in slopes of polarization curves have no general relation to the anode or cathode open - circuit potentials of the corroding system.