Corrosion is one of the major problems encountered in cooling water treatment. Many cooling systems are made from ferrous metal components and two basic problems can result if corrosion is not controlled:
a) Failure of equipment resulting in unscheduled downtime, loss of production and cost of replacements;
b) Decrease in plant efficiency due to deposition of corrosion products in heat exchangers resulting in a loss of heat transfer.
The corrosive potential of a system can be predicted from water analyses and the Langelier or Ryznar indices; empirical tools for predicting scale and corrosive tendencies of natural waters based on their chemical analyses and the thermodynamic constants for CO2/H2O/CaCO3 system. A calculation is made of the theoretical pH (pHs) at which a particular water is just in equilibrium with calcium carbonate. Predictions are then made based on the actual water pHa.
(pHs = (pK2 - pKs) + pCa2 + pAlk )
Langelier Index = pHa - pHs if positive this indicates scale forming, if negative then corrosive.
Ryznar Index = 2pHs - pHa
• 4 - 5 Heavy scale
• 5 - 6 Slight scale
• 6 - 7 Scale / Corrosion
• 7 - 7.5 Corrosive
• 7.5 - 9 Highly corrosive
Protection from corrosion can be achieved either by changing the metal or the environment. Changing the metal is expensive and may not always be successful. Changing the environment is a widely used, practical method to control corrosion. In cooling systems two practical approaches may be adopted:
a) Use the natural calcium and alkalinity in the water to form a protective calcium carbonate layer on the surface;
b) Add corrosion inhibitors.
Neither approach is exclusive and the choice of treatment will be based on an analysis of system requirements and corrosion control required. Most systems today operate at what is called "natural pH". This is the pH to which the system rises at the operating concentration factor and is usually in the rage 8 to 9. This can lead to reduced corrosion rates but there may be still the need for additional corrosion inhibitor to achieve satisfactory corrosion rates
The most widely occurring scale forming chemical is calcium carbonate (CaCO3). Other common scale forming chemicals are calcium sulphate (CaSO4) and calcium phosphate (Ca3(PO4)2. These and other less common chemicals that can form scales such as iron oxides, silica (and silicates) have one property in common - they are either only very slightly soluble or exhibit inverse solubility characteristics, i.e. they become less soluble with rising water temperature.
Scale formation reduces the ability of the metal to conduct heat effectively and leads to a reduction in the efficiency of the system.
Scale control can be achieved in three ways:
1. by operating the system in a non scaling way ( Langelier / Ryznar help define this);
2. by removing some or all the calcium using a water softener;
3. by modifying the precipitation of the scale with inhibitors so that it is non-scale forming.
Again these approaches are not mutually exclusive and treatment is often a combination of all three
The water in a cooling water treatment system provides a favourable environment for the growth of micro-organisms. Microbial growth on surfaces can lead to the development of biofilms. If uncontrolled, such films can cause fouling which can:
• reduce the performance of equipment;
• promote scale formation and metal corrosion;
• create a reservoir of unwanted organisms such as legionella.
Microbial control is most often achieved by the addition of oxidising biocides (either chlorine or bromine) as the prime biocidal treatment that may be complemented by speciality organic biocides and biodispersants.
Deposition Fouling Problems
Deposition, as compared to scale formation, occurs when insoluble particulates in the circulating water adhere to surfaces. Deposition can be a particular problem when flow velocities are low or there are dead ends. Particulate deposition can be the precursor for scale, corrosion and biofilm formation.
Most particulates, such as clay or silt, enter a system with the make-up water. Other sources are through airborne contamination being scrubbed out in the tower (corrosion products from internal corrosion) aluminum and iron hydroxides can enter a system from poorly operated pre-treatment plant.
Deposition Control is achieved by:
• reducing particulates in the make-up water through filtration or sedimentation processes;
• maintaining water velocities to avoid settling;
• ensuring the absence of "dead ends";
• adding dispersants (chemicals which adsorb on particle surfaces and help maintain them in suspension) as part of the cooling water treatment programme.
In the UK, the operation of evaporative cooling systems is regulated by government guidelines designed to prevent these systems becoming sources for the spread of legionella bacteria to the population. The guidelines indicate that:
• the problems described above have to be controlled adequately;
• the system has to be correctly maintained (in particular to avoid the formation of aerosols) and monitored to ensure control is adequate;
• regular cleaning and disinfection are an integral part of system operation.
Adherence to the guidelines will ensure that system operation will not lead to problems with legionella.