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May 1998No. TR-009

White Rust and GalvanizedCooling Towers

Factory assembled cooling Towers have beenmanufactured from heavy mill galvanized (HMG)steel for many years. HMG steel is continuously hotdip coated with protective zinc at the steel process-ing mill. The steel is treated subsequent to coatingwith a chromate rinse to initially passivate the zincsurface, providing for protection during storage,fabrication and initial operation from wet storagestain. In the last several years, there have beenincreasing numbers of reports of steel coolingtowers experiencing premature corrosion, generallytermed white rust.

It is very important the tower owner under-stands that white rust occurs in only a very smallpercentage of galvanized cooling towers, and it ispreventable. Some possible combinations of waterquality and treatment chemicals that might beexperienced in the field could produce the followingscenarios:

Untreated water itself may passivate thegalvanized surface. White rust would not occur inthis situation. However, untreated water may havea detrimental corrosive effect on other systems

metallurgies, i.e. steel pipe and copper heat ex-changer tubing. Consult a competent water treateras described in the following text.

More traditional water treatment programsfor corrosion and scale control may passivate thegalvanized surface. White rust would not occur inthis situation.

Recently introduced, higher alkalinity watertreatment programs may inhibit formation of aprotective film on the galvanized surface, interfering

with the passivation in a small number of cases.Initial and periodic passivation of metal surfacesmay be required.

Even in the worst case situation, white rust ispreventable. It is recommended that a competentwater treatment specialist (familiar with local waterquality and the materials of construction includingzinc/galvanized steel) be consulted, and that aproper water monitoring program be initiated.

Definition of White RustWhite rust is the accumulation of appreciable

volumes of a soft, white, fluffy, non-protective zinccorrosion product on galvanized surfaces. With thisnon-protectiveporous reaction product in place, thesurface is not passive to future zinc reaction andcorrosion continues. Typically, white rust appearssuddenly and progresses rapidly over the wetted,galvanized steel components in the cooling tower,condenser, or cooler. 1 If this condition is notcorrected, it may lead to premature failure of thegalvanized coating.

Possible causes of White RustHMG steel normally provides excellent corro-sion resistance in cooling towers and many otherapplications. Zinc is a very reactive metal which actsboth as a sacrificial anode protecting the steel and asa barrier to continuous ongoing reaction. Maximumservice life for zinc is attained in a neutral pHenvironment (pH 6.5 - 8.0), which allows formationof the non-porous zinc carbonate/zinc hydroxidepassivation barrier on the surface.

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1 Johnson, Keith M. and Michelic, Update on White RustCorrosion and Control, Technical Paper TP91-14, CoolingTower Institute, Houston, Texas (1991).

2 Private correspondence, Mr. David Hudok, Weirton Steel

Corporation, December 12, 1991.

As federal and local regulations increasinglyrestrict the use of many corrosion inhibitors (such aschromates), water treatment systems have come torely heavily on more alkaline (high pH) waterchemistries to help control corrosion rates. Whilethere is some disagreement about the actual cause ofthe recent increase in reported cases of white rust,

various water treatment experts are now concludingthat some of the newer, higher alkalinity, treatmentchemistries actually inhibit the formation of protec-tive films on the galvanized surface and thus allowdevelopment of white rust.

There are also indications that soft water (lessthan 30 ppm total hardness), when combined withthese high pH conditions, can make the problemworse.

Galvanized Steel Quality

One recently published study has unfortu-nately attributed the formation of white rust tochanges in the zinc thickness or increases in thealuminum content of the HDG zinc or galvanizedsteel. In fact, all major suppliers of galvanizedcooling towers use G235 coating, which has a zincthickness of 2.0 mils per side or 2.35 oz. per squarefoot of steel sheet. Due to the need to bend metalduring fabrication, HMG materials do contain smallamounts (less than .3%) of aluminum. This is notnew. One major supplier of galvanized sheet steelstated they have been producing continuous hot

dip galvanized sheet with . . . aluminum . . . since1960. 2 The aluminum is not a new factor in thewhite rust phenomenon. Current HMG materialsused in cooling towers is identical if not superior tothe HMG sheet steel produced in years past.

ConclusionFortunately, white rust can be prevented, but it

is important that the proper precautions are takenwhen a tower first becomes operational. It is nowgenerally accepted that in order to get maximumservice life from a galvanized cooling tower, the wetcontact surfaces must be passivated. In many cases(depending on water chemistry), the galvanizedcoating will be passivated by untreated water. Onother projects, a variety of treatment programswhich passivate and subsequently protect zincsurfaces are available that are more or less suited tolocal water conditions. In some areas, traditional(phosphate type) treatment systems may actuallyhelp passivate the zinc surfaces and preclude whiterust formation. Chemical pretreats to passivate maybe effective, but may require repassivation periodi-cally.

In any event, a competent water treatmentspecialist, familiar with local water conditions, thematerials in your cooling tower and cooling system,and white rust, should be consulted. A proper waterquality monitoring program for system metallurgiesshould be initiated. Ask for a program which willprovide excellent zinc corrosion inhibition, and beaware that concentration levels of treatment chemi-cals within recommended limits is essential.

In highly corrosive environments wheregalvanized steel may not provide the desiredcorrosion resistance, alternate materials of construc-

tion may be more suitable. Contact your localMarley representative for information on which ofthe alternate materials of construction now availablewould best suit your application and site require-ments.

7401 W 129 Street Overland Park, KS 66213 913 664 7400www.marleyct.com email: info@marleyct.spx.comIn the interest of technological progress, all products aresubject to design and/or material change without notice.2003 Marley Cooling Technologies, Inc. Printed in USA