Filming amine use in multi-metal hot water systems
In recent years, there has been growing interest in the use of filming amines in a variety of water treatments applications. Although filming amines have been used for decades in various corrosion inhibition applications, their use as primary corrosion inhibitors in heating and cooling water applications has been limited. The increased use of aluminum heat exchangers in hot water hydronic systems has presented significant challenges to water treatment professionals.
This paper describes work conducted in the laboratory and pilot circulating hot water systems comparing the performance of a filming amine blend to other traditional treatments such as nitrite. Studies were performed in the pH range recommended for aluminum heat exchangers and in the higher range (8.5 to 9.5) consistent with traditional treatments. Results showed the filming amine blend provided equal or better corrosion inhibition in all pH ranges compared to traditional inhibitors in the study.
Corrosion in industrial and commercial water treatment continues to be a concern as companies strive to reduce costs while maintaining assets. The need to protect heat exchange equipment and associated piping is vital in minimizing operational costs and preventing downtime. Traditionally, copper and carbon steel have been the metallurgies of choice for comfort heating and cooling heat exchange equipment. However, aluminum became an attractive option for heat transfer as it has a high thermal conductivity relative to its initial capital cost. While this is an alluring option, it adds challenges to current water treatment programs.
Aluminum is a more anodic metal than iron and copper, increasing its potential for galvanic corrosion, which can lead to devastating effects. Aluminum also is an amphoteric metal with its protective oxide film being soluble at both low and high pH. Because of this, aluminum boiler manufacturers often recommend a lower pH range for optimal protection (8 to 8.5).
This places the water treatment professional in a difficult position, as this raises the corrosion potential for carbon steel, which prefers a higher pH (greater than 9.0). For systems containing aluminum heat exchangers, the need to provide corrosion inhibition for all metal surfaces has never been greater.
Filming amines have been around for decades, and in the water treatment space, are most often used in steam boiler applications. While their use as primary corrosion inhibitors has been limited, they are receiving increased attention in heating and cooling water applications.
Film forming amines (FFAs) are being considered for application in both closed and open cooling water systems, as they are biodegradable and leave no hazardous byproducts. FFAs have a strong affinity for metal surfaces, likely due to the partial positive charge on the nitrogen atom as the amine comes into contact with metal surfaces. This attraction is exacerbated by the hydrophobic tail, further pushing the amine away from water and toward the metal surface, leaving a tightly packed film with hydrophobic tails protruding outward.
Polyamines, which inhibit corrosion in a similar mechanism to monoamines, often are preferred as they can form a more resilient film. Film formation is a dynamic process where constant attachment and detachment is taking place, the additional secondary amine provides an extra binding site between the FFA and the metal surface.
The de-wetting of the metal surface inhibits corrosion by preventing dissolved gases and water from reaching the metal surface.Having a resistant film for corrosion inhibition provides additional leeway for system upsets, or times of suboptimal feeding unlike neutralizing amines, which carbonic acid quickly consumes. The application of filming amines can be tightly controlled when combined with a fluorescent tracer and commercially available filming amine residual tests. The high affinity between filming amines and the metal surface means they are able to penetrate existing deposits and corrosion debris quite well.
When applying FFAs for the first time to systems previously fouled with deposits or corrosion byproducts, it is suggested to start with a low dose and gradually increase the levels. This helps to avoid excessive sloughing off, which can cause blockages in small diameter passages or in low flow areas.
Nitrite acts as a mild steel corrosion inhibitor by oxidizing the metal surface to form a protective Fe2O3 film. However, nitrite provides no aluminum protection, and can even be aggressive to aluminum surfaces.
In lower temperature hot water systems (less than 140°F), nitrite can become susceptible to microbiological growth from both nitrifying and denitrifying bacteria. Nitrite reacts with oxidizing microbicides to form nitrate, and therefore should not be used together.
Molybdate, which can be a bit more costly, is known to provide corrosion protection for both aluminum and mild steel. Unlike nitrite, it is not vulnerable to microbiological degradation, but does require the presence of dissolved oxygen to form its protective oxide layer on mild steel. Protection of aluminum with molybdate is most effective in a relatively tight pH range (7.8 to 8.3).
Silicates are cost efficient corrosion inhibitors and are known to exhibit some corrosion protection for all metals. Similar to molybdate, they are not degraded by microorganisms, and operate in a similar pH range. While silicates can be used for many applications, they are most commonly used as an adjunct to enhance other treatment programs.
Editor’s Note: The second part of this case study will be featured in the next issue of iWWD.