This article, which explores methods for controlling corrosion in cooling water systems, is the third in an occasional series on water management basics and technologies.
Water is relatively inexpensive and is an excellent transporter of heat. It also is an excellent polar solvent and eventually will dissolve just about all known materials. For this reason, the chemistry of all cooling water treatment programs must begin by addressing corrosion.
Corrosion is essentially an electrochemical oxidation process that destroys the basic metals from which most cooling systems are constructed. Uncontrolled corrosion is often responsible for physical equipment failure and the plugging of cooling water passages from corrosive deposits. More subtle effects that are often not linked to corrosion include the loss of production speed or control, as well as decreased energy efficiency from corrosive deposits on heat transfer surfaces, where the deposits act as an insulator and decrease thermal conductivity.
Factors Affecting Corrosion
Many factors affect the expected uncontrolled corrosion rates in a cooling water system. The presence of dissolved gases, chloride and sulfate levels, pH, alkalinity, scaling tendency and protective ions such as phosphate and silicate must all be considered in the design of corrosion control chemistry, along with engineering factors such as water temperatures, the presence of galvanic couples and the water velocity. In most cases, corrosion cannot be eliminated, only minimized or controlled at acceptable rates.
A recently recognized problem with galvanized cooling towers is “white rust,” which is the accelerated corrosion of the zinc coating on galvanized steel. This problem became common when the lead content of the zinc used for galvanizing was reduced due to environmental restrictions, resulting in a more easily corroded zinc alloy. White rust is a more serious problem in cooling systems operated at pH values above 8.2 su.
The amount of white rust corrosion is directly proportional to the total alkalinity and pH of the cooling water -- higher values of either parameter will result in an increased corrosion rate. Control methods include controlling the pH of the cooling water, reducing the cycles of operation, and using a specific zinc-corrosion-inhibitor chemistry. The numerous “pretreatments” recommended by various firms do not control white rust for more than a few weeks after the treatment.
Test Control Method
The effectiveness of the corrosion-inhibitor portion of a cooling water treatment program should be monitored by regularly using corrosion-monitoring coupons to determine actual corrosion rates within the cooling water system. Electronic corrosion-rate meters are often fairly accurate and quite valuable; however, they are substantially more expensive than corrosion coupons and do not provide long-term rate data. While not as good as direct corrosion monitoring, a complete chemical analysis of the makeup and cooling water can be performed by a skilled technician to spot problems such as excessive corrosion of copper alloys or zinc in the system. Corrosion rates above the acceptable levels require immediate remedial action.
A good cooling water treatment program should reduce corrosion rates to the average levels shown in table 1. However, attaining these control levels, which equate to a corrosion rate reduction of 85 to 95 percent over uncontrolled levels, can be difficult due to the factors noted previously.
Purchasing agents have often been quoted as saying, “All water treatment chemicals are the same, so we will buy the cheapest ones.” This statement sums up some of the misinformation that abounds on water treatment chemistry. While it is true that most suppliers have products with similar chemistries and can duplicate each other's products, it is the application of specific products to a particular facility's makeup water and cooling system design that differentiates success from failure.
Many specific and blended chemical inhibitors are commonly used in cooling water treatment programs to control corrosion. Table 2 lists some common inhibitors with related comments.
Most successful cooling water treatment programs use several chemical inhibitors blended into one product to take advantage of a synergistic effect, where the net reduction in corrosion from using the mixture is greater than the sum obtained from the individual components. For example, adding 2 mg/l of zinc to a phosphonate product at 10 mg/l reduced the corrosion rate on mild steel from 2.2 to 0.9 mils/yr. Because of the increase in effectiveness, many programs use mixtures such as molybdate-silicate-azole-polydiol, phosphonate-phosphate-azole and molybdate-phosphonate-polydiol-azole.
For facilities where the cooling water system is constructed of several materials (virtually all industrial facilities), a program using a blended corrosion-inhibitor product is required to obtain satisfactory corrosion protection. The exact inhibitor chemistry must be determined by the water treatment program supplier following an evaluation of the makeup water chemistry, system construction materials and operating conditions.
Many schemes for reusing/recycling treated industrial wastewater, particularly in the electronics industry, use soft water as cooling tower makeup. In these and other cooling systems operated with naturally soft or softened makeup water, controlling corrosion is a significant challenge. The common water treatment industry practice of using makeup water hardness and alkalinity to provide the bulk of corrosion control action does not work with soft makeup water. In fact, the most commonly used corrosion inhibitors, polyphosphates and phosphonates, do not work if less than 50 mg/l calcium hardness is present in the cycled cooling water.
This often-overlooked fact has been responsible for many documented problems where this chemistry was applied to soft water makeup systems with disastrous results. For example, at one glass manufacturing plant in Zanesville, Ohio (which has since closed), using softened makeup water in a cooling tower resulted in the destruction of a compressor intercooler assembly due to corrosion in less than six months. The repair cost, which exceeded $25,000, was small compared to the plant's production losses from shutting down a 1,200-hp unit for two weeks. In a more recent incident, a school in Phoenix, Ariz., started a new HVAC cooling system up with softened makeup water and a typical phosphonate-polymer based treatment. The cooling water was so corrosive that it caused significant damage to the stainless steel cooling tower basin in less than a year.
Because soft makeup water can be used to achieve high cycles or zero blowdown operations for water use reduction or environmental reasons, these types of problems will likely become more common.
Softening a hard water source to make it suitable for use as cooling tower makeup can also increase the potential for white rust corrosion of new galvanized steel components. Due to the normally high level of alkalinity associated with hard water, softening can accelerate white rust corrosion by factors exceeding 100 times. Inspections of new softened makeup cooling tower systems have shown that more than 50 percent of the galvanizing has been removed from the cooling tower in less than 30 days.
Facilities using soft water as cooling system makeup should consult a knowledgeable water treatment program supplier that has demonstrated expertise in the use of softened makeup water. PCE
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