CORROSION AND SCALE CONTROL,FOULING CONTROL,MICROBIAL CONTROL

Corrosion in recirculating cooling water systems is controlled by employing either

inorganic or organic inhibitors. The four major inorganic inhibitors are chromate,

zinc, orthophosphate, and polyphosphate. Minor supplements include molybdate,

nitrite, nitrate, various organic nitrogen compounds, silicate, and natural

organics.

The earliest chemicals for treating recirculating cooling waters were inorganic

polyphosphates and natural organic materials. The concept was to add a small

amount of acid to control the stability index to a slightly scale-forming value.

Organic corrosion inhibitors include organic phosphorus compounds, specific

synthetic polymers, organic nitrogen compounds, and long-chain carboxylic

acids.

Polyphosphate and natural organic materials were added to the program to

provide both corrosion protection and scale inhibition. The scale inhibition

stemmed from the use of the polyphosphate as a threshold treatment. In addition,

the polyphosphate combined with calcium to form a cathodic inhibitor that

reduced the corrosion rate. The natural organic material tended to keep the metal

surface relatively clean and aid the inhibitor in establishing a protective film. It

also dispersed suspended solids, and modified calcium carbonate and tricalcium

phosphate precipitates if they tended to develop on hot surfaces.

The greatest disadvantage of this treatment approach is the reversion of polyphosphate

to orthosphosphate, which can combine with calcium to form calcium

phosphate scale. For this reason, this type of program has evolved into the stabilized

phosphate program. In this treatment, both ortho- and polyphosphate are

used as corrosion inhibitors. To prevent calcium phosphate deposition, the pH i

generally controlled at 7.0 and specific synthetic polymers are added to disperse

and stabilize calcium phosphate.

The next cooling water treatment was chromate, an exceptionally reliable corrosion

inhibitor. Initially, chromate was applied at very high dosages, frequently

in the range of 200 to 300 mg/L as CrO

the pH to between 6 and 7, preventing calcium carbonate from precipitating. This

treatment was quite effective in both scale inhibition and corrosion protection,

but one shortcoming was that pitting attack tended to occur if the chromate residual

became low. It was found that if chromate were combined with other inhibitors,

particularly cathodic types (e.g., zinc and polyphosphate), the chromate level

could be reduced to 20 to 30 mg/L CrO

to 300 mg/L CrO

acid, frequently controlling the pH to 6 to 7. An additional advantage of synergized

chromate was the margin of safety provided against pitting attack should

the chromate be momentarily underfed.

These synergized chromate formulations are still considered among the best

corrosion inhibitors in use today. However, increasing environmental pressures

are forcing the development of innovative synergized chromate formulations that

permit carrying chromate levels in a recirculating system substantially below 10

mg/L CrO

achieve results with this approach the system pH must be controlled precisely,

and dispersants and biocides used to keep the system clean. An obvious limitation

to this approach is that the reservoir of protection available with the higher CrO

4. Acid was added to the system to lower4 with better results than obtained at 2004 used alone. The synergized chromate approach also employed4 while continuing to provide acceptable corrosion protection. To4

levels does not exist. Therefore, process contamination, uncontrolled microbial

activity, fouling, and deposition will disrupt the system much more quickly than

at the more traditional 20 to 30 mg/L CrO

Although chromate has done an outstanding job for years, increasing environmental

concerns have brought pressure on research into new corrosion inhibitors

with potentially less environmental impact. An early result of such research was

the development of organozinc combinations. Since zinc, a cathodic inhibitor,

has a lower film strength than chromate, the pH of the system for an organozinc

program was increased to between 7 and 8 to make the water less corrosive, allowing

the zinc to form a satisfactory inhibitor barrier. The organic portion of the

treatment was a dispersant to keep the system free of deposits, thereby encouraging

formation of an adequate zinc film. In addition to dispersancy, certain types

of organics increased zinc solubility at the higher pH required for this method of

treatment. These programs were adequate in many industrial plants, but because

the inhibitor film at the operating pH was not as effective as a chromate film, these

programs did not substantially replace traditional chromate-type treatments.

Subsequently, an innovative concept in cooling water chemistry arrived with

the introduction of organophosphorus compounds. Like inorganic polyphosphates,

these prevent scale formation by the threshold effect. However, there the

similarity ends; inorganic polyphosphates easily revert to orthophosphates, with

increasing holding time, temperature, and microbiological attack. Organophosphorus

compounds do not revert under normal cooling tower conditions except

under severe microbiological attack. Further, unlike the inorganic polyphosphates,

the organophosphorus compounds are generally able to inhibit precipitation

of calcium carbonate and other scale-forming species at a higher pH and alkalinity

than tolerated by the inorganic polyphosphates. This development opened

the door to what is now known as the alkaline approach to treating cooling water

systems.

The basic treatment concept is to raise the pH of the operating system to 7.5

to 9.0, thereby substantially reducing the natural corrosivity of the recirculating

water. Experience has shown that although the higher pH provides a less corrosive

water, frequently this reduction is not of sufficient magnitude to protect all mild

steel systems, especially mild steel heat exchangers with high heat flux or low flow

velocities. Thus a specific all-organic inhibitor package is required to control corrosion

and scale. In general, all-organic inhibitors combine organic phosphorus

compounds, synthetic polymers, and aromatic azoles. These combinations provide

corrosion control for steel and copper alloys, scale control, and deposit

control.

Another approach to alkaline treatment involves the use of modern scale and

deposit control agents along with more traditional corrosion inhibitors. Organic

phosphorus compounds and polymers can be supplemented with inorganics like

chromate or zinc. These programs can provide the performance of an all-organic

program at a lower cost, where chromate or zinc can be used.

The significant advantage provided by alkaline operation over earlier treatments

is the buffer capacity provided by the water that reduces the impact of system

upsets on performance. Another particular advantage of the alkaline concept

of treatment is the substantial reduction or occasional elimination of acid feed.

This, of course, depends on the chemistry of the system.

Deposit control in cooling water systems is absolutely essential for maintenance

of heat transfer rates. However, control of deposits is often more difficult in alkaline

systems than in lower pH systems. The makeup water may contain dissolved

solids, organic matter, and suspended solids, any of which can contribute to fouling.

A system may become grossly contaminated with microbes; for example,

makeup water with a high BOD, such as a recycled municipal or industrial

effluent, is particularly susceptible to fouling from slime-forming bacteria.

Table 38.5 shows some sources of foulants in a typical recirculating system.

The raw water and air inoculate a system with colloidal organic matter, silt, soluble

iron, and microbes. Hydrogen sulfide, sulfur dioxide, and ammonia may

enter from the plant atmosphere.

The selection of the proper dispersant for any operating system is based on

actual analysis of a deposit. Synthetic organics, including polymers and surfaceactive

agents, are generally applied for dispersing microbial and organic depos

Synthetic polymers such as polyacrylates or polyacrylamides are dispersants for

silt, sand, iron, and other inorganic deposits. These polymers can be tailor-made

by varying the components and molecular weights to maximize dispersant performance

on specific foulants. Organophosphorus compounds, including polyol

esters and phosphonates, are inhibitors for calcium carbonate and calcium sulfate

precipitates. However, once deposits form, any scale removing action by these

dispersants takes place slowly, so the best approach is to prevent the scale from

forming in the first place.

MICROBIAL CONTROL

Microbial deposits present a special case of fouling. Treatment often requires biocides

to kill microbe colonies and dispersants to loosen and wash them away. The

most common biocide employed in all systems is chlorine. In general, chlorine is

the only biocide required in most systems. If applied continuously at a residual

of 0.2 to 0.4 ppm it will provide effective control at all cooling water pH values.

At alkaline pH, the continuous presence of chlorine species in the water will provide

the required microbial killing power because of the infinite contact time

available. In intermittent chlorination, such as utility cooling systems, the chlorine

contacts the microbial organisms for short periods of time. In this case pH

can be more important. Sterilization studies have shown that chlorine kills faster

at pH 7 than above pH 8. This may be due to the greater amount of HOCl present

in the hypochlorite equilibrium at pH 7. Thus slug chlorination may be more

effective at neutral pH because HOCl has a faster killing power than OC1~.

There are problems associated with the use of chlorine. It can react with some

organic materials, particularly phenolic compounds, to form reaction products

that are nonbiodegradable or refractory, presenting potential effluent problems.

Generally speaking, chlorine can be applied to most recirculating systems without

danger of tower lumber delignification if free chlorine residuals do not exceed 1

mg/L. It is seldom necessary to continually carry a free chlorine residual over 0.2

to 0.3 mg/L to control microbial growths in most systems. Bromine is often a

more practical treatment than chlorine because it remains effective at higher pH

values and avoids formation of the kinds of halogenated by-products resulting

from chlorination.

Although chlorine and bromine are excellent killing agents, their performance

can be significantly improved by the use of biodispersants. Biodispersants aid the

toxicant by breaking loose the biofilms and enabling them to contact more microbial

organisms. In cases of gross contamination or loss of toxicant feed, a contingency

nonoxidizing biocide may be required (See Chapter 22).

4 levels.