Applications of Passivators

17.2.2 Applications of Passivators

Chromates are applied mostly as inhibitors for recirculating cooling waters (e.g., of internal combustion engines, rectifi ers, and cooling towers). The concentration of Na 2 CrO 4 used for this purpose is about 0.04 – 0.2%, with the higher concentra- tions being employed at higher temperatures or in fresh waters of chloride con- centration above 10 ppm. The pH is adjusted, if necessary, to 7.5 – 9.5 by addition of NaOH. Periodic analysis (colorimetric) is required to ensure that the concen-

tration remains above the critical level (10 −3 M or 0.016% Na 2 CrO 4 at room

PASSIVATORS

temperature). Combinations of chromates with polyphosphates or other inhibi- tors may permit the concentration of chromates to fall below the critical level. This reduction in chromate concentration results in some sacrifi ce of inhibiting effi ciency, but there may be adequate protection against pitting for the treatment of very large volumes of water employing cooling towers [15] .

Chromates (Cr 6+ ) are toxic and carcinogenic and cause a rash on prolonged contact with the skin, and so they must be used with caution and with full regard to disposal requirements.

Corrosion rates of mild steel as a function of chromate and chloride concen- tration at various temperatures are shown in Table 17.1 [14] . Nitrites are used as inhibitors for antifreeze cooling waters (see Section

18.2.1 ) because, unlike chromates, they have little tendency to react with alcohols or ethylene glycol. They are not so well - suited to cooling tower waters because they are gradually decomposed by bacteria [16] . They are used to inhibit cutting oil – water emulsions employed in the machining of metals (0.1 – 0.2%). In pipe- lines transporting gasoline and other petroleum products, where water is a very minor phase, suffi cient nitrite or chromate solution may be continuously injected to give a 2% concentration in the water phase [17] . In this connection, gasoline is corrosive to steel because, on reaching lower temperatures underground, it releases dissolved water which, in contact with large quantities of oxygen dis-

solved in the gasoline (solubility of O 2 in gasoline is six times that in water), cor- rodes steel, forming voluminous rust products that may clog the line. Sodium nitrite enters the water phase and effectively inhibits rusting. Chromates used for the same purpose have the disadvantage that they tend to react with some con- stituents of the gasoline.

The corrosion rates of steel in contact with water – gasoline mixtures contain- ing increasing amounts of NaNO 2 are listed in Table 17.2 [18] . The minimum amount of NaNO 2 for effective inhibition is 0.06% or 7

× 10 −3 M which, because of impurities present in the water, is higher than the critical concentration in distilled water. Nitrites are inhibitors only above about pH 6.0. In more acid environments, they decompose, forming volatile nitric oxide and nitrogen

T A B L E 17.2. Corrosion Rates of Mild Steel in Sodium Nitrite Solutions Containing Gasoline [18]

Rotating Bottle Tests Using Pipeline Water, pH 9, and Gasoline; 14 - Day Exposure, Room Temperature

% NaNO 2 Corrosion Rate (mm/y)

310 INHIBITORS AND PASSIVATORS

T A B L E 17.3. Critical Concentrations of NaCl or Na 2 SO 4 above which Pitting of Armco Iron in Chromate or Nitrite Solutions Occurs [11]

5 - Day Tests, 25 ° C, Stagnant Solutions

Critical Concentrations

NaNO 2 50 ppm

> 2000 450 a See also Refs. 4 and 5 .

peroxide. In common with other passivators, they tend to induce pitting at con-

SO 2 4 − ions. In this regard, nitrates are less sensitive to Cl − than to SO 2 4 − , contrary to the situation for chro- mates [3 – 5] (Table 17.3 ).

centrations near the critical in presence of Cl − or