Technology Application Bulletin Control of White Rust with “GalvaGard”
Background
Shortages of fresh water, increasing costs for water and wastewater disposal, and stringent environmental requirements on discharges have created a desire to minimize both water usage and wastewater discharge. One common and very effective method to decrease both water usage and wastewater discharge is to increase the cycles of concentration (COC) at which cooling towers, one of the largest water users, are operated.
Increasing the COC at which a cooling tower is operated increases the pH and alkalinity of the recirculated water. Operation of cooling towers at these higher pH and alkalinity values has resulted in the recognition of a new form of corrosion “White Rust”.
White rust is the common name applied to zinc corrosion; the products of corrosion typically appear as a white to dirty gray voluminous deposit below the water line on galvanized steel surfaces exposed to the recirculated water. The white color is due to the formation of zinc carbonate, which does not form a corrosion limiting protective film on the base metal. Due to this lack of protective film formation, white rust corrosion will proceed until the protective zinc is entirely removed from the underlying steel, which is then subjected to accelerated corrosion (red rust) and premature failure. White rust has been a substantial corrosion problem in some cooling towers, causing equipment failure in as little as three years.
White rust is not really a new phenomenon; it occurs anytime zinc, or galvanized steel, is exposed to water that has a pH value above 8.2. The rate of corrosion, however, is governed by the alkalinity of the water and presence of any accelerating agents, such as phosphates and phosphonates. Higher alkalinity, and the presence of accelerating agents, substantially increases the corrosion rate. In the past where pH control via acid feed was used for scale control, chromium was the corrosion inhibitor of choice, and lower COC; white rust was rarely seen and thus not recognized as a specific problem. With the safety and control problems attendant with use of acid, the USEPA ban on chromium based water treatments, and general increase in COC to conserve water, many more cooling towers are now operated with alkaline water chemistry, putting them into the pH/alkalinity ranges that produce accelerated white rust.
To add to the problem, around 1989 the USEPA began to force zinc galvanizing plants to change the composition of the molten metal “bath” used to produce galvanized steel by reducing the level of lead in the bath to trace levels. The resultant reduction in lead content of galvanized steel produced by the new low lead baths is believed to render it even more susceptible to white rust corrosion in alkaline water.
Problem
Once the problem presented by white rust was recognized, water treatment companies attempted to control it by using acid addition to lower the pH below the critical 8.2 point, using higher levels of traditional corrosion inhibitors, pretreatment passivation of the cooling tower with various phosphate based products, and reducing COC. Cooling tower manufacturers have responded by providing cooling tower fabricated of stainless steel, fiberglass, and plastic; eliminating use of galvanized steel.
Unfortunately, none of these “cures” have been successful in providing a simple, cost effective and reliable cure for this problem while maintaining higher COC. Feeding acid to cooling towers still presents the safety and control problems it always has, with the additional problem that pH adjustment with the popular all-organic cooling water chemistries is critical, just a little too much acid and steel corrosion rates go through the roof. Laboratory examination of the traditional corrosion inhibitors, like phosphates and phosphonates, has shown that increased levels of these materials actually accelerate white rust corrosion, while pretreatment passivation simply does not work. Of course, reducing COC is counterproductive as to reduction of water use and wastewater discharge. Another aspect of COC reduction is that it also substantially increases the use of water treatment chemicals. Needless to say, the cooling towers fabricated from corrosion resistant materials are substantially more costly than galvanized units.
Solution
Recognizing the need for a simple, reliable cure for the white rust problem, ProChemTech initiated research into new corrosion inhibitors. These efforts were successful in that a new inhibitor technology, GalvaGard™, was discovered in 1994 after two years of research. This unique technology is available in a wide array of ProChemTech formulations suitable for makeup waters ranging from soft to extremely hard and with any level of alkalinity.
GalvaGard has also been formulated into an adjunct product for use with existing water management programs and is also available as a concentrate for use by toll product blenders and other water management firms.
ProChemTech International, Inc. is headquartered in Brockway, PA and maintains manufacturing facilities there and in Apache Junction, AZ. The firm specializes in design, engineering, and manufacture of wastewater treatment and reuse systems; formulation and manufacture of specialty chemical products for boiler, cooling, process, and wastewater treatment; provision of complete water management programs; and supply of cooling tower systems.
ProChemTech International, Inc.
Innovation in Water Management
Apache Junction, AZ and Brockway, PA 15824
480-983-5385 www.prochemtech.com 814-265-0959
Look for the GalvaGard label on your cooling water treatment products to insure against white rust corrosion.