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 constructed/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 components. This fact attests to the cost-effectiveness of galvanized steel, and when properly maintained 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 problem that has led to many towers requiring premature replacement. White rust corrosion can reduce 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 corrosion cell is formed. The activity of such a corrosion. 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 provide guidance for those who will consider purchasing and operating a new galvanized steel cooling component or have purchased and need to operate an existing galvanized cooling component.

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 process and water chemistry can impact white rust potential is useful in obtaining a resolution or ideally 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. Some galva- metal, depends on forming and maintaining a nized steel is not suitability for cooling wa- stable and passive oxide layer. ter/HVAC applications. The tower manufacturer needs to ensure that the galvanized steel product If the oxide is disrupted, repair is crucial. If the purchased is suitable for these applications. oxide layer is constantly disrupted or removed, general corrosion potential will increase or in the Up until the 1960’s, the predominant method of case of galvanized steel, depletion of the zinc galvanizing for manufacture of galvanized steel coating will eventually occur. And if pitting corro- cooling towers and other cooling components sion occurs and is not mitigated, the life expec- was a post-fabrication hot dip process. This tancy of the component will be greatly reduced. method of hot-dip galvanizing (HDG) is still used extensively for coating large structural parts (i.e., It is not the intention with this document to detail pre-fabricated cooling tower structural parts, the specific reactions and chemistry of white rust. evaporative condenser bundles, etc.) and for small miscellaneous parts. This zinc coating is It is important to know that the specific mecha- 2 nisms and causes of white rust can vary from rough and heavy (1.5 oz./ft ) with an average system to system since there are a number of thickness of 3 – 6 mils applied to the exposed variables (with various combinations and permu- surface (per side). The galvanizing process often tations) that lead to white rust corrosion. One var- will include a water-based quenching step where iable is the galvanizing process; several changes post-passivation is done, typically using chro- have been noted that have likely reduced the mate. The chromate passivation provides pre- window of tolerance of the galvanized steel to operational protection of the galvanized coating. white rust corrosion. Another variable is water The governing specification for post-fabrication treatment chemistry, which has changed signifi- hot dip galvanizing is ASTM A123. cantly since the early 1980’s.

4 of 16 Three cooling tower OEMs and one trade publica- Chromate is an excellent passivator of galvanized tion6 report that the more common galvanized steel and the reduction or elimination, in some steel product used today for cooling tower manu- cases, of chromate is expected to increase the facture is the heavy mill galvanizing (HMG) pro- vulnerability of the galvanized steel to white rust. cess. Water Chemistry & Treatment This is also a hot-dip process, but instead of post- fabrication & batch galvanizing, the raw, pre- A typical water treatment program is designed to fabricated rolled steel sheet is put through a con- control scale, corrosion and microbiological relat- tinuous galvanizing process. The galvanized ed problems that may occur throughout the cool- sheet roll still needs to be cold-worked by the ing cycle. The old standard of using chromate- tower OEM for fabrication of cooling towers; based treatments and acid pH control along with hence, this can be termed a pre-fabrication pro- a biocide provided excellent results. This treat- cess. The governing specification for pre- ment and pH chemistry regime were favorable to fabricated hot-dip galvanizing is ASTM A653 (al- protecting and maintaining galvanized steel sur- so, cooling tower components should meet a faces, but is long gone due to regulatory ban of G210 HMG classification). The HMG process will chromates in the 1980’s. produce a more uniform, thinner coating of zinc and zinc-steel interalloy (relative to the post- Today’s cooling water treatment programs have manufacture galvanizing process) with at least been greatly influenced by several factors includ- 3.0 mils thickness (2.1 oz./ft2) total or 1.5 mils ing environmental restrictions, energy and water (1.05 oz./ft2) on each side. Aluminum may be conservation efforts, and the on-going focus on added primarily to enhance the corrosion re- increasing facility safety. Some specific factors sistance of this thinner coating. Quenching may include: be either an air-cooled or water-spray process.  As noted, the USEPA ban of chromates in Chromate post-passivation may be done or some cooling systems - effectively implemented by other form of pre-operational protection may be the middle 1980’s, used. Electrogalvanizing is a third galvanizing process  A more recent and growing trend toward re- where zinc is deposited on steel in a relatively ducing the concentration of phosphate-based thin layer by a process of electroplating. There is inhibitors, no interalloy layering with this process and the  The use of acids has grown less popular due weight of zinc applied is thin compared to hot-dip to safety and handling concerns, galvanizing. Consequently, electrogalvanized steel product would have a fairly short life expec-  Efforts to conserve water and/or reduce oper- tancy if used for the manufacture of wetted cool- ating costs have pushed many operations to ing tower parts. increased cycling of the water chemistry, Experience indicates that both HDG and HMG  In many cases, the facility will modify the wa- galvanized steel can provide reliable, long-term ter source to achieve higher cycles or use operating service in a cooling tower environment. poorer quality water sources, which are lower However, as reported in at least two publica- cost and/or more plentiful. 8,11 tions , there are notable differences between Consequently, water treatment professionals the HDG and HMG methods of galvanizing (and have adopted and supported these trends by resulting product) that can directly impact the ini- modifying the water treatment program. Today, tial tolerance to white rust corrosion and generally many treatments are using less anodic corrosion impact the life expectancy of galvanized steel inhibitors and have compensated with a higher cooling components. It should not be assumed pH control range in order to provide effective cor- that all galvanized steel product has equal toler- rosion control and avoid acid feed. Water soften- ance to white rust corrosion. For example, due to ing has become a more common option to help more stringent environmental regulation, some maximize water conservation. Unfortunately, galvanized steel producers no longer use chro- these trends have mostly been contrary to the mate passivation while others have reduced the needs of protecting and maintaining galvanized concentration of chromate in their passivation steel surfaces. step.

5 of 16 The following section will highlight the needs for Pre-Installation Handling Guidelines: the chemical treatment program and provide wa-  Abide by the American Galvanizers Association ter chemistry guidelines that can help ensure rea- recommendation to store galvanized metals sonable life expectancy for all cooling system under dry conditions until it is placed in service components, including galvanized steel compo- to prevent “wet storage staining”. nents. The following section should also help a prospective buyer (of a cooling tower) to deter-  Tower manufacturer publications may or may mine if galvanized steel is an appropriate material not note if the galvanized steel is pre- of construction choice. passivated with chromate. The manufacturer’s product should be pre-passivated with chro- Section Two - Prevention of White Rust mate or some suitable alternative should be uti- lized. The discussion of white rust corrosion prevention is presented to address the responsibilities of the  Several cooling tower OEMs note a need to equipment OEM and that of the water treater consider alternative materials of construction separately. It is critical that the personnel specify- (MOC) if system conditions are expected or ing, purchasing and ultimately operating the cool- known to be harsh relative to galvanized steel. ing system be educated on what the require- The choice of cooling tower construction mate- ments are for the prevention of white rust. rials should consider corrosion resistance, structural integrity and durability, desired If these requirements cannot be achieved, an equipment life, and not just upfront cost. Stain- alternate cooling component material of con- less steel, plastic, fiberglass and epoxy coated struction should be considered (see Section galvanized are becoming common alternatives Five). to galvanized steel, but at a higher upfront cost, to gain improved equipment life. Equipment Manufacturers’ Perspective Post-Installation Handling Guidelines: Cooling equipment OEMs have the responsibility to manufacture a product that meets customer  All OEM publications reviewed indicate that the and industry specifications. To help ensure the potential for white rust corrosion is greatest product achieves life expectancy, cooling equip- when the tower is newly constructed, having a ment manufacturers have developed chemistry freshly exposed galvanized surface. All OEM and water treatment recommendations for cooling companies referenced below recommend the towers and related equipment. The seller, buyer tower be pre-passivated prior to putting any and owner/operator needs to ensure that the in- heat load on the tower. tended or existing conditions will be able to  All OEM publications reviewed indicate that achieve the manufacturer’s recommendations. proper water chemistry and chemical treatment The information to follow is extracted from several during initial tower start-up is essential to the in- cooling equipment manufacturer references. The itial formation of a passive zinc oxide. In par- specific manufacturers whose documents were ticular, alkalinity/pH control and the presence of reviewed are identified in the Table 1. Moreover, calcium hardness are emphasized. these recommended operating ranges are sum- marized in Figure 1 – Galvanized Towers Operat-  All OEM publications reviewed emphasize the ing Ranges. This visually differentiates between need to have a water treatment professional, initial and routine service. knowledgeable of the topic of white rust prevention, involved in the start-up and operating process.

6 of 16 TABLE 1 COOLING TOWER MANUFACTURERS - RECOMMENDED WATER CHEMISTRIES Parameter BAC2 Evapco7 Marley12 OEM Reference: BAC Operating Manual Evapco Eng. Bulletin 036A Manual 92-114B Passivation Duration: 4 to 8 weeks 4 to 12 weeks Minimum of 8 weeks. pH during Passivation: >7.0 to <8.2 >7.0 – <8.0 >6.5 – <8.0 pH for Routine Service: 6.5 to 9.0 suggested >6.0 to <9.0 No specific guide found

Hardness (as CaCO3): >30 ppm >50 ppm 100 – 300 ppm

Alkalinity (as CaCO3): <500 ppm <300 ppm 100 – 300 ppm

Chlorides (as Cl ): <250 ppm <250 ppm No specific guide found

Sulfates (as SO4): <250 ppm <250 ppm No specific guide found Conductivity: <2,400 µS <2,400 µS No specific guide found

Chlorine (as Free CL2): <1.0 ppm as a routine <0.5 ppm as a routine No specific guide found  BAC offers removal &  Critical to have a pas-  Chromate rinse used General Comments: treatment recommenda- sivation plan and assigned for HMG steel sheet. tions for white rust. responsibilities prior start-up. NOTE: Cooling OEMs suggest that the system be initially treated with the maximum allowable level of a non-oxidizing biocide and/or sodium hypochlorite (oxidizing biocide) to a level of 4 to 5 mg/l free chlorine at a pH of 7.0 to 7.6. This recommendation is in place as a sound practice for bacteria control in cooling towers (see AWT Position paper on Legionella Guidelines for further discussion of this important issue). Exposure of galvanized steel to elevated chlorine level will increase corrosion potential of new, unpassivated galvanized steel and may damage passivated galvanized steel. Figure 1 - Galvanized Towers Operating Ranges 5 300

6 250 Initial

7 200

pH Service 8 150 Routine Total Hardness

9 100

Service 10 50

7 of 16 Water Treatment Companies’ Perspective Tower Preconditioning Phase Check List: Since the water treatment professional is often in  Clean and passivate any newly installed cool- the position of being a unit operations consultant, ing system or component prior to or upon ini- it is important they be aware and communicate tial exposure to circulating water. Galvanized the established industry knowledge for maintain- steel surfaces have the same requirement to ing galvanized steel tower surfaces. Also, the wa- be cleaned and passivated as other metals, ter treatment professional can help by communi- such as steel, but offer some special limita- cating (to the specifying company and/or the tions. During the initial startup phase is when owner/operator) the likely consequences when white rust is most likely to occur and conse- these water chemistry and operating require- quently impact on the life expectancy of the ments are not maintained. galvanized steel tower or cooling related Too often the decisions dealing with design, in- component. The startup phase may last sev- stallation and start-up of a new cooling tower and eral days to accommodate the “system”, but related equipment are made without any or insuf- passivation of the “galvanized component(s)” ficient review and input from a qualified water may require several weeks to several months treatment professional. Consequently, the re- to achieve desired results. quirements for a trouble-free, long operating life  Control pH/alkalinity during the initial expo- of the galvanized steel tower are compromised. sure of the galvanized surface to recirculating In today’s competitive environment it is increas- water: between pH 7.0 to 8.0 being ideal. ingly critical that the owner/operator protect and Cleaners should be buffered to maintain pH optimize their investment. Involving a knowledge- between 6.5 and 8.0. The water treater able water treatment professional early on in the should be capable of selecting an appropriate review process will help minimize problems and cleaner, but typically a phosphate-based will help optimize the owner/operator’s invest- and/or silicate-based cleaner is used. Specif- ment. ically, inorganic phosphates are typically used This section will discuss the critical considera- for passivation. An acidic phosphate (such as tions a water treatment consultant should consid- phosphoric acid) can aid the conditioning pro- er and communicate during the tower- cess and help buffer the pH. Note each mg/L preconditioning phase, during the routine operat- of phosphoric acid, as PO4, will neutralize ing phase and during any idle operation/lay-up roughly 0.5 to 0.7 mg/L of bicarbonate alkalin- phase. ity. Phosphate addition can range from 10’s of mg/L to 100’s of mg/L concentration. Howev- There are some basic established requirements er, one should consider calcium phosphate that ideally should be assessed before deciding deposition potential before applying the high on the purchase/use of a galvanized steel tower. phosphate residuals. These include: 1) raw water chemistry parame- ters such as alkalinity, calcium hardness, chlo-  Use an effective copper corrosion inhibitor rides and sulfates, 2) will the makeup be sof- that will minimize the copper level in the recir- tened, 3) can the galvanized component be iso- culating water and complex any soluble cop- lated from the system and 4) can the galvanized per to minimize potential for re-deposition. surface be properly passivated prior to heat load High copper levels in the circulating water can being applied. Generally speaking, if the existing re-deposit on metal surfaces, particularly gal- system conditions make it difficult to effectively vanized metal. accommodate the needs for maintaining galva-  Isolate the fresh galvanized surface from any nized steel, then one should reconsider the pur- harsh solutions/cleaners. There may be pre- chase and installation of a galvanized steel tower. existing parts of the system that require Refer to Section Five in this document, which will strong acidic or alkaline conditions. Ideally, address alternative material of construction selec- the system design allows for the galvanized tion. equipment to be isolated and by-passed.

8 of 16  A minimum calcium of 30 to 100+ mg/L (as of acid are critical and must be considered to en- CaCO3) is desired to achieve proper pas- sure operator and system safety. sivation when using phosphate-based or  Control tower chemistry, considering treat- phosphate/molybdate-based treatments. ment capability, to minimize corrosion poten- Temporary addition of calcium may be re- tial of steel, copper (if present) and galva- quired (i.e., bypassing a makeup water sof- nized steel. Care must be taken with the wa- tener or by addition of a calcium source). ter treatment not to harm the galvanized steel.  Halogen products should not be routinely fed  Overfeed of phosphonates, polymers and to exceed 1.0 mg/L free halogen (as Cl ). 2 other chelating chemistries should be avoid- However, it is understood that proper sanitiza- ed. If the galvanized oxide is harmed, recon- tion may require up to 10 mg/L free halogen ditioning of the galvanized steel surface (as as Cl for a period of 24 hours. Passivation af- 2 identified in the preconditioning phase) may ter sanitization may be required. be required. Remove white rust by reducing  Avoid starting-up a new unpassivated galva- the pH below 8.0, preferably to neutral pH, nized steel component with full heat load. and implementing an effective treatment Heat load on the system during precleaning clean-up program (physical and chemical) and passivation should be minimized or ideal- targeted for galvanized steel. ly avoided to prevent concentration of salts  Add maintenance chemicals ensuring they and minimize corrosion potential. are well mixed and diluted prior to contact  Monitor the galvanized surface prior to and with the galvanized surface. As a rule, avoid during preconditioning. Monitoring should in- adding chemical treatment directly to the clude as a minimum, visual inspection and tray/sump if constructed of galvanized steel. documentation. Monitoring may also include In the case where system upsets may require trending zinc in the makeup and recirculating harsh chemicals to be used, the galvanized water to assess zinc oxide pick-up. Corrosion component should be isolated from the water coupons or a corrosion rate probe have been circulation or an appropriate galvanized steel used with some success. Monitoring will be inhibitor used. covered in Section Five of this document.  On-going visual monitoring of the tower’s gal-

vanized steel surface should be a service visit Routine Operating Phase Check List: routine. Other forms of monitoring may be

useful and will be covered later in this docu- Once the tower and system is precleaned and ment. passivated, the water chemistry and operating

conditions can be modified to accommodate pro- Idle Operating/Lay-up Phase Check List: cess needs. However, there will still be limitations

that should be considered for galvanized steel An “operating” system in many ways is much eas- components. ier to treat and protect than an “idle” cooling sys-  The tower pH may exceed 8.0; however, it is tem and/or tower. However, for various reasons recommended the pH be increased slowly cooling systems and/or cooling tower(s) will need (not all at once) to the intended target. A pH to be shutdown. of 9.0 is a desired maximum; although some  Lay-up solutions should be buffered to main- tower treatments can allow a pH greater than tain pH between 7.0 and 9.0. Excessive 9.0 (consult with the water treatment repre- pH/alkalinity can destroy the protective zinc sentative servicing the facility). If excessive oxide and result in white rust corrosion. Note, pH is identified as a concern, the own- it is most common that the “cooling tower” er/operator should plan to operate at lower component will be drained during lay-up. cycles or control pH/alkalinity with acid feed or by dealkalizing the source makeup water.  Cleaning and passivation may be required to accommodate special issues such as system Note operating at lower cycles is costly and may sanitization. For example, sanitization may be precluded due to blowdown limitations. Re- require high levels of halogen (i.e., > 10 mg/L quirements for proper handling, feed, and control halogen, as Cl2) after an extended shutdown.

9 of 16 Repassivation may be required after the sani- Following there are two primary chemical ap- tization. proaches that will be reviewed reportedly capable of controlling white rust with reactive inhibitors.  If at all possible, water circulation through the system should not be shutdown. Ideally, by- The first approach is analogous to that of control- pass the tower completely or least by-pass ling mild steel corrosion where blends of common the tower fill. inhibitors such as molybdate, phosphate, phosphonates, polyphosphates, zinc and/or other Section Three: White Rust Treatment Basics compounds believed to work are added to the system using proprietary formulae. There are var- Evolving Technologies ious reports, industry papers, patents, etc that Most water treatment professionals have access show data claiming efficacy for such treatments. to conventional inhibitor technologies capable of While it is not the position of AWT to endorse any maintaining low steel and copper corrosion rates. specific treatment, the approach of limiting anodic These conventional technologies are often ade- and/or cathodic reactions involved in the destruc- quate where galvanized steel is used if the galva- tion of the galvanized surface is a valid approach. nized steel surface has been properly seasoned. However, there does not appear to be a consen- Some newer technologies are being used and sus or even a leading series of guidelines within have been promoted as having enhanced capa- the water treatment community to support a par- bility to protect galvanized surface3,15. It should be ticular combination of inhibitors or formula. the goal of the general water treatment communi- The second approach is the use of strong ligands ty to gain a better understanding of these tech- to react with solubilized zinc to form a complex nologies and to continue to develop promising that has limited or no reactivity with hydroxide or technologies for galvanized steel. This section will carbonate ions. One class of chemicals with re- review the basis for some of these technologies. ported success is dithiocarbamates.

Dithiocarbamates are sulfur compounds prepared Common Treatment Approaches from the reaction of amines with carbon disulfide. The management of white rust in process water The resultant dithiocarbamate compound can can provoke a wide range of treatment approach- form highly water insoluble complexes with most es. While no one approach is considered stand- transition metals. Because of this property, dithio- ard, three primary treatment themes emerge carbamates are well suited to complex with zinc when field practices are examined: Passivation, ions at the water/surface interface and limit the Water Chemistry Control and the use of Reactive ability of the metal ion to subsequently complex Inhibitors. Passivation and Water Chemistry Con- with either hydroxyl or carbonate ions that are trol have already been addressed in Section 2. necessary to form white rust. The specific com- The two approaches remain the most commonly position of dithiocarbamates can vary widely de- used concepts in mitigating or at least controlling pending upon the starting amine and, as such, a white rust. Reactive inhibitors represent the use range of dithiocarbamates have reported efficacy of treatment concepts that reportedly mitigate or in the literature. The addition of other compounds control white rust and broaden the operating such as phosphonates and molybdate are report- chemistry window tolerable to avoid white rust. ed to show significant improvements over the use Reactive Inhibitors of dithiocarbamates alone. Reactive inhibitors refer to those chemical components that are specifically added in order to Section Four - Removal & Repair of White Rust limit the reactions involved in the formation of Removal of White Rust white rust. As opposed to protective film formation or water chemistry control, reactive inhibi- Whether or not to remove the White Rust? tors are designed to either slow anodic or cathod- As noted, white rust corrosion is characterized as ic reactions or complex with zinc ions as they are a localized/pitting type corrosion and identified by liberated from the metal surface to prevent sub- characteristic white, waxy tubercule-type depos- sequent reaction with free carbonate and hydrox- its. ide ions.

10 of 16 However, not all white deposits found on galva- Repairing Damaged Galvanized Surfaces nized steel surface are due to white rust and not Re-galvanizing with Zinc-Rich Paints: all deposits, including zinc-rich deposits, will result in localized/pitting corrosion.  ZRC Worldwide ZRC Cold Galvanizing Compound Consequently, it is incumbent of the owner/operator, with guidance from the water treat-  Sherwin Williams ment professional, to determine if the deposits Zinc Clad XI are better left alone or if removal is required.  Benjamin Moore & Co. Epoxy Zinc Rich Primer CM18/19 Evaluation can include any or all of the following: Non-galvanic finishes  Deposit analysis – determine the inorganic content. It may be the deposits are calcium-  Belzona, Inc. based and not zinc oxide. Belzona 1111 Supermetal Belzona 5811 Immersion Grade  Physical inspection of the surface under the deposits – investigate to determine if there is  Benjamin Moore & Co. pitting corrosion resulting beneath the depos- Coal Tar Epoxy M47/48 it. Consider leaving the deposit alone if pitting Low Cure Epoxy Mastic Coating M45L/46 is not observed.  PPG Industries  Age of the equipment and of the deposits – COAL CAT Amine-Cured Coal Tar Epoxy the deposits may be doing more good than COAL CAT Resinous Cured Coal Tar Epoxy harm. A tower that is far along in life expectancy with white rust that has been present for Please Note: The above list of manufacturers is years is probably better off left alone. not meant to represent a complete list of coating suppliers nor is meant to be an endorsement of Mechanical Cleaning Methods: these products. Virtually all information recommends the removal of the white rust by brushing with a stiff (non- Application Guidelines: metallic) bristle brush and then coating the dam- To achieve reasonable performance from the aged areas. If the white rust build-up is light or post-installation finishes, it is critical to properly spotty, it should be easily brushed off to allow for prepare the surface. This will require removing the formation of the protective zinc oxide. This debris and deposits, cleaning the base surface process can be enhanced by the addition of inor- (typically with a phosphate-based cleaner) and ganic phosphate or by the reduction of the repairing any areas where failure has occurred. pH/alkalinity during the repassivation step. The surface should be dry before applying the Chemical Cleaning Methods: finish. There are products that may be applied to In mild cases the area should be brushed (using a wet surface; however, results are usually tem- a stiff non-metallic bristle brush) with a mild porary. Best results will typically be achieved by cleaning solution. Severe cases may require mul- having a professional, experienced in this trade of tiple applications of a more concentrated cleaning metal surface finishes, perform the task. solution along with brushing. Phosphoric acid is an excellent choice, although other acids such as Application instructions will vary somewhat acetic, glycolic or citric acid have been used with among manufacturers - the basic steps are: success. 1. Remove sealing compound from corners. Care should be taken when using these other acids since they can chelate the base zinc coating. 2. Sandblast surface to near-white profile. Grind- Overzealous application of such chelating agents ing the surface and wire brushing the rusted may strip the zinc coating from the steel surface. areas may be acceptable, but not as effective Follow the cleaning process with a thorough wa- as sandblasting. ter rinse.

11 of 16 3. Completely remove debris, clean and dry sur- Section Five - Monitoring for White Rust face - use fans to promote faster drying. Historically physical inspection, mass balance 4. Apply coating according to manufacturer’s in- and galvanized steel coupons have been used to struction; typically two coats are required to at- ascertain if white rust corrosion of galvanized tain a minimum desired dry film thickness. steel was an ongoing problem in cooling towers. 5. Allow fully coated surface to dry/cure for speci- This section will provide some thoughts and guid- fied time period (it can be as short as 1 to 3 ance to the value and methodology nuances of days and as long as 14 days with zinc-rich each. paints). In some cases, application and curing The simplest method of monitoring is visual ob- times may be accelerated – check with coating servation of the galvanized surface. Physical in- manufacturer. spection of cooling towers for white rust has Notes: proven to be fairly reliable in that white rust forms an easily identified soft white, waxy deposit on 1) Zinc rich compounds require extended cure galvanized surfaces which when removed shows times (up to 14 days) in order to provide the best a definite area of attack on the metal surface. possible performance. However, visual inspection is not preventative or 2) The most important factors for the success of proactive and it may not allow for the detection of paint systems are adhesion and continuity – and zinc coating loss, unless gross loss occurs. in the case of zinc-rich paints, electrical conductivity. Standard mass balance analysis of cooling wa- ters to determine if a white rust problem is occur- Continuity of the paint systems is extremely im- ring is generally not usable as the corroded zinc portant for carbon steel, since pinholes and other will deposit as corrosion product (i.e., white rust) imperfections quickly become rust pits. Zinc-rich and thus is not measured in the cooling water. paints must be electrically conductive in order to The exception to this is where sufficiently aggres- provide cathodic protection. sive treatments are used such that zinc is dis- 3) Surfaces to be reconditioned, which will be solved into cooling water. This may occur during subject to immersion should be prepared in ac- the initial start-up phase, routine operational cordance with Near White Metal Blast SSPC- phase or during post-operational cleaning events. SP10. In these cases, measuring zinc in the makeup and recirculating water (and factoring tower cy- Refer to ASTM Section A 780 for details on these cles) to determine zinc pick-up can help monitor and other approved repair methods for galva- zinc oxide stability. For example, based on zinc nized steel surfaces. measurement (i.e. zincblowdown/zincmakeup), one can determine if zinc pick-up is occurring at the ex- Leak repair pense of the galvanized steel surface. It is ex- Quick fix (for sump/pan area): insert a stainless pected that zinc oxide pick-up may be high to steel bolt through the hole with a rubber gasket start, but it should level off with time and eventu- on each side of the affected area. The use of tar ally approach theoretical tower cycles. Note, this or an epoxy can help seal this type of repair. For monitoring method will not be effective if zinc is larger areas use a piece of plastic sheet, fasten fed as part of the treatment program. with rivets and use tar or epoxy to achieve a seal. Corrosion coupons and to a lesser extent corro- Long term repair (for sump/pan area): some cool- sion probes outfitted with zinc coated tips have ing tower OEMs will provide a retrofit fiberglass long been used to monitor for white rust corro- basin. The cost of the sump insert is not a signif- sion. Two material options to consider when us- icant expense, but the cost of installation can be ing corrosion coupons are hot dip galvanized expensive since the tower may require partial steel or pure zinc corrosion coupons. dismantling.

12 of 16 Table 2 – Example Coupon Data data sources and over time as a trend for each Min. & Max Range, individual operating cooling system. Makeup Water mpy loss Average, Quality, mg/L mpy loss In the absence of any formally presented data, it Min. Max. is still unclear how to fully interpret the result of TH <5, TA 91 5.49 32.55 11.62 zinc-based corrosion coupon data. There is an Galvanized Coupon 1.35 8.03 AWT reference17 that offers a good overview of TH 75, TA15 2.00 5.83 3.09 best practices for corrosion coupon monitoring. This same project committee is currently looking Galvanized Coupon 0.49 1.44 to establish a specific performance rating system TH 44, TA 52 1.53 3.98 2.65 relating to galvanized coupon corrosion rates. Galvanized Coupon 0.38 0.98 Once complete, that specific data will be provid- TH 241, TA 235 1.9 5.7 3.23 ed. Galvanized Coupon 0.47 1.41 Section Six - Alternative Materials Data is strictly provided as example. This section will endeavor to provide some guid- The field data shown is provided as an example to ance on whether the (cooling) system conditions highlight the difference in data obtained from pure represent a high risk for shortened life expectan- zinc coupons versus the HDG coupons. It sug- cy (see figure 2) and will offer suggestions on al- gests that because there is less zinc to be re- ternative materials (see Table 4). Included is a moved from the HDG coupon as compared to a summary of the basic features and benefits of pure zinc corrosion coupon that under the same alternative materials and some review of their lim- conditions the reported metal loss will be lower itations. The most popular alternative MOC (perhaps understating the actual corrosion result). choice to galvanized steel cooling towers and evaporative condensers is a hybrid of stainless There also needs to be some rational given as to steel/galvanized steel or all stainless steel (ex- when this monitoring approach is best suited. For cluding fill, distribution nozzles and louvers). Fi- example, the use of corrosion coupons is ideal berglass and plastic are gaining somewhat in when monitoring galvanized surface during sys- popularity, but are still a high cost option, espe- tem start-up and pre & post operational cleaning cially when structural integrity is fortified. events. Furthermore, this monitoring method will work well when using “reactive” white rust inhibi- Selection of Galvanized Steel Material tors as part of a maintenance treatment regiment. The decision tree shown on the following page However, there are some practical issues that will offer guidance as to whether galvanized steel need to be considered when using zinc-based should be selected. This decision tree is simply a coupons to monitor a maintenance treatment that guide and should not be used to draw absolute does not employ a “reactive” white rust inhibitor conclusions as to whether galvanized steel MOC regiment. For example, if the coupon is not pre- is the right choice or the wrong choice for a par- passivated, then the water chemistry of the circu- ticular application. lating cooling water will have a predominant affect on the coupon regardless of what affect the same Table 4 that follows offers a basic overview of the water source may have on the actual zinc sur- alternative materials one may want to consider if faced equipment. This is because an already the risk assessment guide suggests that galva- “passivated” galvanized metal surface is more nized steel is not appropriate for existing or ex- tolerant of a broader chemistry window. An un- pected application conditions. Each of the alter- passivated zinc-based corrosion coupon exposed native materials may have advantage(s) over gal- to water chemistry under heat load, operating vanized steel; however, the reader is encouraged conditions will demonstrate white rust, but will not to pay close attention to the limitations noted for necessarily reflect accurately what is happening these alternative materials as well. to the equipment that has been passivated. When in doubt, it is best to consult with one or As with all corrosion coupon data, this coupon more tower OEMs and water treatment consult- data needs to be evaluated in the context of other ants.

13 of 16 FIGURE 2 Decision Tree - Okay To Use Galvanized?

Startup and Tower will not be yes operating Yes yes Will pH meet target exposed to harsh OKAY specs naturally? chemistries meet chemicals? specs naturally?

No No No

pH can be yes Startup and operating yes Tower can be yes OKAY adjusted to meet chemistries can be isolated from harsh target specs? adjusted to meet specs? chemicals?

No No No

Consider alternate Consider alternate Consider alternate material material material

Table Summary: Stainless steel is among the fastest growing al- As one might expect, tower materials with longer ternative materials used, replacing galvanized life expectancy will tend to have a higher relative steel. A stainless steel hybrid with galvanized cost. Galvanized and epoxy coated galvanized steel is a common trend as well. The hybrid tow- steel towers have the lowest life expectancy, but er considers the structural components of signifi- offer a relatively low-cost option. cant vulnerability for galvanized steel and replac- es these components with stainless steel. Stain- If conditions are abusive, the life expectancy of less steel can be vulnerable to chloride pitting any of the materials shown above may be short- and to stress corrosion cracking (although chlo- ened. However, it is fair to say that the more ex- ride tolerance is typically greater than that re- pensive materials are more forgiving. It holds quired for galvanized steel). Stainless steel type, true that galvanized steel has a narrower window temperature, chloride concentration and surface of tolerance. cleanliness are all important factors when using this material. Epoxy coated galvanized steel is offered by one OEM. The OEM claims that the epoxy coating in Fiberglass continues to gain ground as an alter- combination with the base galvanized steel effec- native material, but cost remains an issue and tively protects the base steel substrate. If the structural integrity can be a limiting factor to size epoxy coating is disrupted the exposed galva- of cooling application. Relatively new manufac- nized steel will become quite anodic (corrosion turing technique for high-strength structural com- will be localized to this small exposed area) and ponents will address the structural integrity issue, white rust-type corrosion is likely to occur. but cost continues to be an issue.

Consequently, the epoxy coated galvanized steel Wood and concrete materials remain common- is considered to be only a minor upgrade at best place in medium-sized to large cooling towers from galvanized steel. applications. However, these materials are not commonly used as an alternative to galvanized The seller and buyer should inquire with the steel. Wood has been a viable alternative in the manufacturer as to whether this epoxy coating past for smaller cooling applications, but wood can effectively expand the window of tolerance material is not readily available today and wood for operating and chemistry conditions considered has a fire concern. to be non-conforming for galvanized steel.

14 of 16 TABLE 4 Life Expectancy (expected Cost Factor Limitations/Comments Material Type & Uses Vs. (galvanized = 1.0X) (ease of use) theoretical) Ceramic ● Weight can be an issue 20+ Vs. 30+ - Tower structure 2.5 to 3.0X+ ● Fill more prone to fouling years - Tower fill Fiberglass/Plastic ● Prone to UV degradation - Tower structure ● Structural integrity can be a 2.5X for small tower - Dist. deck & basin 15-20 Vs. 25+ limitation to size >2.5X to increase - Tower fill & louvers years ● Fastener material can be a structural integrity weak link ● Generally easy to fabricate Wood ● Availability of wood product - Tower structure 20+ Vs. 30+ ● Prone to MB degradation 3.0X+ - Tower fill & louvers years ● Can be fire hazard concern - Distribution deck Stainless Steel ● Avoid high chlorides 15-20 Vs. 25+ - Tower structure 1.8X to 2.0X ● Keep surface clean years - Distribution deck ● Generally easy to fabricate Concrete ● Weight, roof-top installations 20-25 Vs. 30+ - Tower structure 3.0X+ ● Rebar corrosion years - Tower basin ● Generally easy to use Epoxy Coated ● Maintain coating to protect gal- Galvanized Steel vanized surface - Tower structure 10-15 Vs. 20+ ● Avoid high chlorides and sul- 1.1X to 1.2X - Dist. deck & basin years fates - Louvers ● Typical coating life is 2 to 10 years per AWT survey Galvanized Steel ● Prone to white rust - Tower structure ● Proper startup conditions are 10 Vs. 15-20 - Dist. deck & basin 1.0X critical years - Louvers ● Avoid chemistry upsets. - Evap. condenser ● Generally easy to fabricate

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Section Seven - Reference List

1. American Galvanizers Association. (1997). Wet storage stain. T-WSS-97. http://www.galvanizeit.org/images/uploads/publicationPDFs/Wet_Storage_Stain_On_Galvanized_Steel.pdf

2. BaltimoreAircoil Company. (1992). White rust on galvanized steel cooling towers. (Excerpted from reference 5.) http://www.baltimoreaircoil.com/english/resource-library/file/806

3. Busch, B. D. & Oldsberg, M. T. (The Analyst, Summer, 2000). Advances in the inhibition of white rust corrosion.

4. CTI Manual, Chapter 9 - Materials of Construction for Cooling Towers (October 2009) http://www.cti.org/pub/ctimanual.shtml

5. Johnson, Keith M. and Joseph B. Mihehc, “Update on White Rust Corrosion and Control”, Technical Pa- per TP91-14, Cooling Tower Institute, Houston, TX (1991) http://www.cti.org/tech_papers/corrosion.php

6. CTI. (1992). Guidelines for treatment of galvanized cooling towers to prevent white rust. PFM-142.

7. EVAPCO. (2009). White rust. Engineering Bulletin No. 036A. http://www.evapco.com/sites/evapco.com/files/white_papers/36-White-Rust.pdf

8. Mayer, W. F., & Larsen, R. (1991). White rust report. Associated Laboratories.

9. Johnson, K. M. & Milelic, J. B. (1990). Diagnosing white rust corrosion in cooling tower systems. NACE Paper No. 361. (Classified as Historic, Available by direct phone call to NACE).

10. Kunz, R. G. & Hines, D. W. (1990). Corrosion of zinc in cooling water. NACE Paper No. 348. (See Ref 9)

11. Laronge, T. (1991). The white rust problem in cooling towers: A technical review. Thomas Laronge Re- port.

12. Marley Cooling Tower Company. (1992). White rust & water treatment. Manual 92-1184B. http://spxcooling.com/pdf/M92-1184B.pdf

13. Materials Performance. (1995, October). Coating basics: Coating newly galvanized steel. (Classified as Historic, See Ref 9)

14. McKay, R. J. & Association of Water Technologies. (2000). Survey of AWT members on white rust. McLean, VA: The Association of Water Technologies.

15. Puckorius & Associates, Inc. (2001, Third Quarter). Cooling water systems – Passivation. WaterChem Solutions, 5.

16. Rachels, G. K. (The Analyst, Spring, 1991). White rust – The water treater’s achilles heel.

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