HOT- AND COLD -WATER TREATMENT

18.2 HOT- AND COLD -WATER TREATMENT

1. Hot - Water Heating Systems. These are closed steel systems in which the initial corrosion of the system soon consumes dissolved oxygen; corrosion is negligible once the dissolved oxygen is consumed. A continuing minor reaction of steel with water produces hydrogen, with a characteristic odor due to traces of hydrocarbon gases originating from the reaction of car- bides in steel with water. The hydrogen reaction can be minimized by

treating the water with NaOH or Na 3 PO 4 to a pH of 8.5.

2. Municipal Water Supplies. Usually, hard waters of positive saturation index are relatively noncorrosive and do not require treatment of any kind for corrosion control. Soft waters, on the other hand, cause rapid accu- mulation of rust in ferrous piping, are readily contaminated with toxic quantities of lead salts on passing through lead piping, and cause blue staining of bathroom fi xtures by copper salts originating from slight cor- rosion of copper or brass piping.

Chemical treatment of potable waters is limited to small concentrations of inexpensive, nontoxic chemicals, such as polyphosphates, orthophosphates, and silicates. Zinc polyphosphate has been used for more than 60 years in controlling corrosion of steel in municipal water systems. Zinc orthophosphate is effective in controlling corrosion of steel, copper, and lead.

Raising the saturation index is a potentially useful means for reducing the corrosion rate in either fl owing or stagnant portions of the distribution system, and it is also effective for reducing corrosion of hot - water systems. This treatment

requires addition of lime [Ca(OH) 2 ] or both lime and soda ash (Na 2 CO 3 ) to the water in amounts that raise the saturation index to about +0.5 (see Section

7.2.6.1 ). For the treatment to be successful, the water must be low in colloidal matter and in dissolved solids other than calcium salts. Corrosion of copper, lead, and brass is also reduced by this treatment. In hot - water systems, the possibility

of excess deposition of CaCO 3 , which causes scaling, must be taken into account in arriving at the proper proportions of added chemicals. Sodium silicate treatment in the amount of about 4 – 15 ppm SiO 2 is sometimes used by individual owners of buildings in soft - water areas. This treatment reduces “ red water ” caused by suspended rust resulting from corrosion of ferrous piping, and it also eliminates blue staining by water that has passed through copper or brass piping.

The conditions under which protection exists or is optimum are not entirely understood, but it is clear that dissolved calcium and magnesium salts have an effect and that some protection may result alone from the alkaline properties of sodium silicate. In the presence of silicate, passivity of iron may be observed at pH 10 with an accompanying reduction of the corrosion rate to 0.1 – 0.7 gmd [7] . Sodium hydroxide induces similar passivity and corresponding low corrosion rates at the somewhat higher pH range of 10 – 11. Under other conditions (e.g., pH 8), a protective diffusion - barrier fi lm is formed, apparently containing SiO 2

322 TREATMENT OF WATER AND STEAM SYSTEMS

and consisting perhaps of an insoluble iron silicate. Laboratory tests in distilled water at 25 ° C showed a reduction in the corrosion rate of iron on the order of

85 – 90% when sodium silicate was added to bring the pH to 8 [7] . However, no inhibition was obtained in the laboratory at the same SiO 2 content using Cam- bridge tap water (pH 8.3, 44 ppm Ca, 10 ppm Mg, 16 ppm Cl − ). If larger quantitites of sodium silicate were added to raise the pH of the water to 10 or 11, the range in which passivity of iron occurs, a marked decrease in the corrosion rate was observed.

Domestic or industrial hot - water heaters of galvanized steel through which hot aerated water passes continuously are not protected reliably in all types of water by nontoxic chemical additions such as silicates or polyphosphates. Adjust- ment of the saturation index to a more positive value, as discussed earlier, is sometimes helpful. Often, cathodic protection or use of nonferrous metals, such as copper or 70% Ni – Cu (Monel), is the best or only practical measure.

18.2.1 Cooling Waters

Once - through cooling waters (usually obtained from rivers, lakes, or wells) usually cannot be treated chemically, both because of the large quantities of inhibitors required and because of the problem of water pollution. Sometimes, additions of about 2 – 5 ppm sodium or calcium polyphosphate are made to help reduce corrosion of steel equipment. In such small concentrations, polyphos- phates are not toxic, but water disposal may continue to be a problem because of the need to avoid accumulation of phosphates in rivers and lakes. Adjustment of the saturation index to a more positive value is sometimes a practical possibil- ity. Otherwise, a protective coating or metals more corrosion resistant than steel must be used.

Recirculating cooling waters, as for engine - cooling systems, may be treated with sodium chromate (Na 2 CrO 4 ) in the amount of 0.04 – 0.2% (or the equivalent amount of Na 2 Cr 2 O 7 · 2H 2 O plus alkali to pH 8). Chromates inhibit corrosion of steel, copper, brass, aluminum, and soldered components of such systems. Since chromate is consumed slowly, additions must be made periodically in order to maintain the concentration above the critical. For diesel or other heavy - duty engines, 2000 ppm sodium chromate (0.2%) can be used to reduce damage by cavitation - erosion as well as by aqueous corrosion (see Section 7.2.5.1 ). Because of the toxicity of hexavalent chromium (Cr +6 ), chromates must be handled, used, and disposed of appropriately and with regard to regulatory requirements.

Chromates cannot be used in the presence of antifreeze solutions, because of their tendency to react with organic substances. Many proprietary inhibitor mixtures are on the market that are usually dissolved beforehand in methanol or in ethylene glycol in order to simplify the packaging problem, but this also limits the available number of suitable inhibitors. In the United States, borax

(Na 2 B 4 O 7 · 10H 2 O) is a common ingredient. Sulfonated oils, which produce an oily protective coating, and mercaptobenzothiazole, which specifi cally inhibits corrosion of copper and at the same time removes the accelerating infl uence of

BOILER-WATER TREATMENT

dissolved Cu 2+ on corrosion of other portions of the system, are sometimes added to borax. One specifi cation calls for a fi nal concentration in the antifreeze

solution of 1.7% borax, 0.1% mercaptobenzothiazole, and 0.06% Na 2 HPO 4 , with the latter being added specifi cally to protect aluminum. Ethanolamine phos- phate is also used as an inhibitor for engine - cooling systems containing ethylene glycol.

For industrial water cooled by recirculation through a spray or tray - type tower, chromates are the most reliable from the standpoint of effi cient inhibition. However, the critical concentration is high; and as the sulfate and chloride con- centrations increase through evaporation of the water, chromates tend to cause pitting or may cause increased galvanic effects at dissimilar metal couples. Windage losses (loss of spray by wind) must be carefully avoided because chro- mates are toxic. Toxicity also makes it diffi cult to dispose of chromate solutions whenever it becomes necessary to reduce the concentration of accumulated chlorides and sulfates.

Because of pollution problems that can be caused by chromates, numerous inhibitor systems have been developed as alternatives — for example, organic phosphonic acids, which are effective in alkaline waters and are biodegradable

[8] . Nontoxic inhibitor formulations containing mixtures of azoles and water - soluble phosphates (e.g., disodium phosphate and sodium tripolyphosphate) have been developed [9] . Sodium molybdate, which is less toxic than sodium chromate, has also been reported to be a useful component of inhibitor formula- tions for use in recirculating aqueous systems [9, 10] . Other nontoxic, chromate -

free inhibitor formulations are based on mixtures of sorbitol, benzotriazole or tolytriazole, and water - soluble phosphates [9] . Sodium polyphosphate is often used in a concentration of about 10 – 100 ppm, sometimes with added zinc salts to improve inhibition. The pH value is adjusted to 5 – 6 in order to minimize pitting and tubercle formation, as well as scale depo- sition. Polyphosphates decompose slowly into orthophosphates, which, in the presence of calcium or magnesium ions, precipitate insoluble calcium or magne- sium orthophosphate, causing scale formation on the warmer parts of the system. Unlike chromates, they favor algae growth, which necessitates the addition of algaecides to the water. Corrosion inhibition with polyphosphates is less effective than that by chromates, but polyphosphates in low concentration are not toxic, and the required optimum amount of inhibitor is less than that for chromates.