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White Rust Prevention

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White Rust Prevention White Rust Prevention An Industry Update and Guide Paper - 2012 Presented By: Association of Water Technologies (AWT) New Galvanizing Passivated Galvanizing White Rust 1 of 16 Special Acknowledgements The on-going occurrence of white rust corrosion This is a revision of the original 2002 document of cooling-related components led the AWT version. A product of the White Rust Project Technical Committee to create a White Rust team, this document has been updated by the Project team and conduct a survey amongst the Cooling Technical Committee and Special Pro- AWT membership to assess the magnitude of jects Committee of AWT. Special thanks is given concern for white rust corrosion. A brief overview to the Technical Committee Chairs and AWT of the survey results is as follows: white rust cor- Board of Directors for their gracious contribution rosion was identified as a serious and prevalent of time and knowledge toward the production problem. It was identified that white rust corro- and updating of this document. sion occurs predominantly with newly construct- ed/installed galvanized steel towers and related Warning and Disclaimer cooling components. The predominant chemistry This document is designed to provide infor- parameter known to aggravate white rust is high mation regarding the subject matter presented. alkalinity/high pH, and is further aggravated by It is produced with the understanding that neither low hardness (softened water) and/or elevated AWT nor the authors (or other contributors) is chloride and sulfate concentration. It is known rendering legal, medical, engineering, or other that water treatment professionals have various professional services. Neither AWT nor the au- methods of prevention, but that these methods thors (or other contributors) shall be liable for are not always successful when alkalinity/pH, damages, in any event, for incidental or conse- chlorides, sulfates and/or hardness levels are quential damages caused, or alleged to be not maintained within the prescribed ranges. caused, directly or indirectly, by the use of any information disclosed in this document, including Furthermore, the conclusions of the survey of- the use of any recommendations, methods, fered the following: 1) white rust is a prevalent products, services, instructions, or ideas. problem and 2) the AWT organization should prepare a topic update and guidelines to in- Forward crease awareness and promote prevention of The Association of Water Technologies (AWT) is white rust corrosion of galvanized steel cooling an international trade association founded to components. The intention of this publication is serve the interests of water treatment profes- to draw from and summarize published refer- sionals and to advance the technologies of safe, ences and anecdotal experiences into one cen- sound and responsible water treatment practice. tral document that will effectively present the top- AWT is a non-profit organization providing edu- ic of white rust corrosion and its prevention. The cation and training, public awareness, network- intended audiences for this document are water ing, research, industry standards and resource treatment professionals, cooling tower own- support. Association activities serve to benefit ers/operators, and architect/design and mechan- members, as well as advance the arts and sci- ical contracting firms involved in the specification ences of the water treatment industry. Moreover, and/or installation of cooling-related compo- AWT makes a commitment to the public as a nents. Prevention of white rust corrosion can be responsible steward of the environment. accomplished if all parties involved in specifying, manufacturing, operating and maintaining galva- The corrosion of galvanized steel components nized steel cooling components work together. commonly used in HVAC-related applications, Reference sources are provided for more de- such as cooling towers and evaporative conden- tailed information on the causes, cures and pre- sers, may be referred to as white rust and the vention of white rust corrosion of galvanized consequence of white rust can be premature steel cooling towers and related galvanized steel failure of galvanized steel components. cooling equipment. 2 of 16 Section One - Introduction and Background White Rust Galvanizing produces a coating of zinc-iron in- Since the 1950’s, galvanized steel has remained termetallic alloy layers on steel with a relatively the principal material of construction for factory pure outer layer of zinc. assembled cooling towers and related compo- nents. This fact attests to the cost-effectiveness of galvanized steel, and when properly main- tained this material can provide 20 years or more life expectancy in cooling applications. However, as noted in the Forward of this document, white rust corrosion continues to be a prevalent prob- lem that has led to many towers requiring prema- ture replacement. White rust corrosion can re- duce life expectancy significantly, in some cases failure has occurred within a year or two of startup. This has led to a growing trend of using The zinc is anodic to steel and thus will provide alternative materials of construction for factory cathodic or sacrificial protection to any small are- assembled cooling towers such as fiberglass, as of steel that may be exposed (i.e., scratches, plastic and stainless steel or hybrids of these two cut edges, etc.). Additionally, the zinc coating will materials along with galvanized steel. None the oxidize and provide a physical barrier in protect- less, galvanized steel cooling components still ing the bulk of the steel surface from any direct remain the most common choice especially when contact with the environment. Since the wear of the decision is solely based on up-front cost for galvanized steel in service is inevitable, it is fair to cooling component material. One objective for say that with all things being equal, a thicker (as this document is to offer the reader some guid- measured by weight of zinc applied per surface ance in determining what materials of construc- area) and more durable zinc coating inherently tion might be best based on the water chemistry, will provide protection for a longer period of time. design, environmental and operational conditions existing or expected. White Rust may sometimes be interchanged with Many documents dedicated to the discussion of the term Wet Storage Staining since they have a white rust corrosion have been published over the similar corrosion mechanism. Wet storage stain- last 10-15 years. Some publications8,11 have re- ing is typically a pre-construction problem where ported that changes to the galvanizing and finish- new galvanized steel sheet or parts are exposed ing process has increased the potential for white to a wet or moist environment because of im- rust, while other publications2,5,7,12 refute this con- proper storage. Post-construction white rust is a clusion altogether and report that changes to the problem where the fresh galvanized surface is not water treatment and related cooling water chem- able to form a protective, non-porous basic zinc istry has increased the potential for white rust. oxide and typically the surface is partially wetted Still other documents note that changes to both or completely submerged in water. In both cases, the galvanizing process and the water chemistry the deterioration begins when a localized corro- have increased the potential for white rust corro- sion cell is formed. The activity of such a corro- sion. There will be discussion of both these varia- sion cell/pit, results in rapid penetration through bles later, but briefly; there have been notable the zinc coating to the steel. changes to the galvanizing process and the water treatment chemistry that have been driven in large part by environmental restrictions and regu- lations as well as cost-reduction initiatives. Also, the intent of this document is to identify these manufacturing and treatment changes and pro- vide guidance for those who will consider pur- chasing and operating a new galvanized steel cooling component or have purchased and need to operate an existing galvanized cooling com- ponent. 3 of 16 Under these corrosive conditions, the surrounding The incidence of white rust corrosion can be zinc coating may be unable to protect the base heavily impacted by water chemistry, especially steel and consequently the corrosion will continue during the initial start-up operational period. to penetrate through the base steel. Having awareness as to how the galvanizing pro- cess and water chemistry can impact white rust potential is useful in obtaining a resolution or ide- ally an avoidance of the white rust corrosion. Galvanizing Processes Hot dip galvanizing is applied to a weight per square foot requirement, which can range from light to heavy. The amount of galvanizing applied may also be expressed in terms of thickness, which will correlate with weight, i.e., light/thin to heavy/thick. The hot dip coating actually alloys with the steel and forms an integral zinc-steel al- White rust corrosion is often identified by the loy bond between the base steel and outer pure white, gelatinous or waxy deposit that can be ob- zinc layer. The zinc oxide weight applied, the served. This deposit is a zinc-rich oxide, report- thickness applied to the working surface and in- teralloying are critical factors affecting galvanized edly 3Zn(OH)2 ZnCO3 H2O and can be quite similar chemically to the protective zinc oxide typ- steel performance. Components manufactured for ically identified as a dull-gray passive oxide. One cooling tower application may be manufactured critical difference between the two oxides is that using a post-fabricated hot dip process or a pre- the white rust oxide is porous and generally non- fabricated hot dip process. Another consideration protective of the substrate, while the passive ox- for the galvanized coating relative to performance ide is dense and non-porous effectively protecting is formability. Pre-fabricated hot-dip galvanizing the substrate from exposure to the environment. must allow for cold working to be done without Corrosion control of galvanized steel, as with any damage or fracturing of the coating.
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