Corrosion inhibitors: General

A corrosion inhibitor is any substance that effectively decreases the corrosion rate when added to an environment. An inhibitor can be identified most accurately in relation to its function: removal of the corrosive substance, passivation, precipitation or adsorption. Passivating (anodic) inhibitors form a protective oxide film on the metal surface. For example, the ferrous ions formed due to dissolution of iron in the anodic reaction

FIG. 20. A tube shell type of heat exchanger.

Fe → Fe²⁺ + 2e are oxidized to ferric ions or react with anions such as OH^–, present in solution, to form oxides such as Fe₂O₃ and protect the metal. These inhibitors are the best inhibitors because they can be used in economical concentrations, and their protective films are tenacious and tend to be rapidly repaired if damaged. Precipitating (cathodic) inhibitors are simply chemicals that form insoluble precipitates that can coat and protect the surface.

Precipitated films are not as tenacious as passive films and take longer to repair after an upset. Chemical adsorption of the inhibitor specie on the metal surface can distinctly alter the corrosion susceptibility of the metal. A small film of the inhibitor provides the protection by effectively forming an inert film. Removal of the inhibitor by precipitation of it from water is straightforward.

Examples of passivation inhibitors (anodic inhibitors) include chromate, nitrite, molybdate, and orthophosphate. All are oxidizers and promote passivation by increasing the electrical potential of the iron.

Chromate and nitrite do not require oxygen and can therefore be the most effective. Chromate is an excellent aqueous corrosion inhibitor, particularly from a cost perspective. However, due to health and environmental concerns, use of chromates has been outlawed in many countries, and it is not recommended. Nitrite is also an effective inhibitor, but in open systems it tends to be oxidized to nitrate. Both molybdate and orthophosphate are excellent passivators in the presence of oxygen. Molybdate can be a very effective inhibitor, especially when combined with other chemicals. Its main drawback is its high cost. Orthophosphate is not an oxidizer per se, but becomes one in the presence of oxygen. A negative attribute of orthophosphate is its tendency to precipitate with calcium hardness found in natural waters. In recent years, deposit control agents that prevent this deposition have been developed. Due to its relatively low cost, orthophosphate is widely used as an industrial corrosion inhibitor [65].

The local pH at the cathode of the corrosion cell is elevated due to the generation of hydroxide ions.

Precipitating inhibitors form complexes which are insoluble at this high pH (1–2 pH units above bulk water), but whose deposition can be controlled at the bulk water pH (typically pH7–9). A good example is zinc, which can precipitate as hydroxide, carbonate or phosphate.

2H₂O + O₂ + 4e^– → 4OH^– (46)

2Zn²⁺ + 4OH^– → 2Zn(OH)₂ (47)

Calcium carbonate and calcium orthophosphate are also precipitating inhibitors. Orthophosphate thus exhibits a dual mechanism, acting as both an anodic passivator and a cathodic precipitator.

4.4.6.1. Chromates and zinc-chromate inhibitors

Sodium and potassium chromates and dichromates have been widely used in the past as corrosion inhibitors in cooling water systems. Chromates were probably the most efficient corrosion inhibitors, but since the late 1990s, increased concerns about health and environmental effects related to chromates have severely restricted or prohibited its use in most industrialized countries. Therefore, the use of chromates is not recommended; the following is scientific information on its chemistry, for the reader’s reference.

Chromates help to form oxides that inhibit corrosion according to:

2Fe + 2Na₂CrO₄ + 2H₂O → Fe₂O₃ + Cr₂O₃ + 4NaOH (48)

In order to be used as the only inhibitor compound, chromates need a minimum concentration (in the water) of 300 ppm. The normal concentration level is 500 to 1000 ppm in cooling waters with pH in the range of 7.5–8.5.

However, some cases with very high concentrations, on the order of even 10 000 ppm, have been documented. It was demonstrated that the use of chromates along with zinc salts is more effective in combating corrosion in cooling water systems, using much lower concentrations of both compounds, 50 ppm of chromate and 10 ppm of zinc. The pH of the water in this case can be between 5.8 and 7.0. It should be pointed out that using only chromate with concentrations below 200 ppm (without zinc) could lead to corrosion instead of preventing it [64].

In closed cooling water systems, where different metals could be in contact with the circulating water, chromate concentrations of the order of 10 000 ppm inhibits galvanic corrosion. Very high chromate concentrations protect steels, cast iron, zinc, brass etc., but in systems where the water contains H₂S, chromate treatments become ineffective due to reduction of chromium, according to Eq. (49):

2CrO₄^2– + 3H₂S + 10H⁺→ 2Cr³⁺ + 3S + 8 H₂O (49) 4.4.6.2. Phosphate/phosphonate inhibitors

Many early corrosion treatment programmes used polyphosphate at relatively high levels. In water, polyphosphate undergoes a process of hydrolysis, commonly called ‘reversion’, which returns it to its orthophosphate state. In early programmes, this process often resulted in calcium orthophosphate deposition. Later improvements used combinations of ortho-, poly-, and organic phosphates. The general treatment ranges were:

orthophosphate, polyphosphate and phosphonate with concentrations in the range 2–10 ppm and pH of 6.5–8.5.

A more specific set of control limits within these ranges was developed, based on individual water characteristics and system operating conditions. Where low calcium waters were used (i.e. less than 75 ppm), zinc was often added to provide the desired corrosion protection. With close control of phosphate levels, pH and cycles, it was possible to achieve satisfactory corrosion protection with minimal deposition. However, there was little margin for error, and calcium phosphate deposition was frequently a problem.

The Dianodic II is a trademark treatment system from General Electric that was introduced in the market in 1979. This is a very efficient programme that uses relatively high levels of orthophosphate to promote a protective oxide film on mild steel surfaces, providing superior corrosion inhibition. The use of high phosphate levels was made possible by the development of superior acrylate based copolymers. These polymers are capable of keeping high levels of orthophosphate in solution under typical cooling water conditions, eliminating the problem of calcium phosphate deposition encountered with previous programmes. Dianodic II is an industry standard in non-chromate treatment. The general control ranges for Dianodic II are: total inorganic phosphate 10–25 ppm; calcium as CaCO₃ 75–1200 ppm and pH 6.8–7.8. A recent upgrade in performance and capability has been made with the introduction of Dianodic III, the new halogen stable neutral pH treatment programme of General Electric Water and Process Technologies.

4.4.6.3. Alkaline treatment

There are several advantages when operating a cooling system in an alkaline pH range of 8.0–9.2. First, the water is inherently less corrosive than at lower pH. Second, feed of sulphuric acid (to adjust pH) can be minimized or even eliminated, depending on the make-up water chemistry and desired cycles. A system using this make-up could run an alkaline treatment programme in the 4–10 cycle range with no acid feed. This eliminates the high cost of properly maintaining an acid feed system, along with the safety hazards and handling problems associated with acid. Even if acid cannot be eliminated, there is still an advantage to alkaline operation. A pH of 8.0–9.0 corresponds to an alkalinity range more than twice that of pH7.0–8.0. Therefore, pH is more easily controlled at higher pH, and the higher alkalinity provides more buffering capacity in the event of acid overfeed. A disadvantage of alkaline operation is the increased potential to form calcium carbonate and other calcium and magnesium based scales. This can limit cycles of concentration and require the utilization of deposit control agents.

Alkaline zinc programmes are very effective programmes and rely on a combination of zinc and organic phosphate (phosphonate) for corrosion inhibition. Zinc is an excellent cathodic inhibitor that allows operation at lower calcium and alkalinity levels than other alkaline treatments. However, discharge of cooling tower blow down containing zinc may be severely limited due to its aquatic toxicity. Zinc based programmes are most applicable in plants where zinc can be removed in the waste treatment process.

Alkaline phosphate programmes using organic and inorganic phosphates also inhibit corrosion at alkaline pH.

Superior synthetic polymer technology has been applied to eliminate many of the fouling problems encountered with early phosphate/phosphonate programmes. Because of the higher pH and alkalinity, the required phosphate levels are lower than in Dianodic II treatments. General treatment ranges are: inorganic phosphate, 2–10 ppm;

organic phosphate, 3–8 ppm; calcium (as CaCO₃), 75–1200 ppm; and pH8.0–9.2.

All-organic programmes use no inorganic phosphates or zinc. Corrosion protection is provided by phosphonates and organic film-forming inhibitors. These programmes typically require a pH range of 8.7–9.2 to take advantage of calcium carbonate as a cathodic inhibitor.

4.4.6.4. Nitrites

Nitrites are excellent for passivating steels and other iron based alloys. When used to treat distilled cooling waters, 50 ppm of nitrites is sufficient to inhibit corrosion. However, in the presence of high concentrations of sodium chloride, about 5000 ppm of sodium nitrite is recommended and the pH maintained around 6.5–8.0.

Protection of the metal is through formation of oxides according to:

2Fe + NaNO₂ + 2H₂O → Fe₂O₃ + NaOH + NH₃ (50)

4.4.6.5. Quaternary salts of ammonia

Quaternary salts of ammonia protect metals in cooling systems by forming films on the metal surfaces. The use of 10–15 ppm of these salts in cooling water systems has been found to be effective to protect metals from corrosion.

4.4.6.6. Molybdate based treatment

In order to be effective, molybdate alone treatment requires very high concentrations. Therefore, it is usually applied at lower levels (e.g. 2–20 ppm) and combined with other inhibitors, such as inorganic and organic phosphates. Many investigators believe that molybdate, at the mentioned levels, is effective in controlling pitting on mild steel. Because molybdate is more expensive than most conventional corrosion inhibitors on a parts per million basis, the benefit of molybdate addition must be weighed against the incremental cost. Use of molybdate may be most appropriate where phosphate and/or zinc discharge is limited.