brochure - faq pipe galvanizing

The PipeGalvanizing Process

Frequently Asked Questions



About this Booklet

This booklet describes the process and addresses a range of FrequentlyAsked Questions (FAQs) about automated galvanizing of steel pipe andtube. Detailed information on specific topics is contained in a series ofAppendices.

While the fundamental galvanizing process has changed little since itsinception, engineering developments in the galvanizing of pipe and tubehave taken place to make it more automatic in operation, and incontrolling the thickness and smoothness of galvanized coatings.

Although the overall process has remained essentially the same,operators and managers of pipe galvanizing plants change for variousreasons. And while new operators and management might undertaketraining, some may still have questions about the galvanizing processand methods used for galvanizing pipe.

This booklet deals specifically with galvanizing of pipe and tube in plantsdesigned for this purpose. It does not cover in-line (continuous)galvanizing or general (batch) galvanizing, specifically. However, someinformation may be applicable to the latter. The booklet lists some of themost frequently asked questions and provides answers and explanations.Where possible, the questions have been grouped according toprocessing step.

DISCLAIMER:

Teck Cominco Metals Ltd. makes no representations or claims as to the suitability of this information for yourparticular purpose, and that to the extent you use or implement this information in your own setting, you do soat your own risk. The information provided in this booklet is solely for your own use and informational purposes.In no event will Teck Cominco Metals Ltd. be liable for any damages whatsoever, whether direct, consequential,incidental or special arising out of the use of or inability to use the information provided in this booklet.



Table of Contents

About this Booklet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

The ProcessWhat is pipe galvanizing? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4What are the basic steps for galvanizing pipe? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Surface Preparation - PicklingWhich acid should be used? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6How can pickling acid be controlled? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

Surface Preparation - FluxingWhy is flux used and how are pipes fluxed? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7How can flux solution be controlled? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Galvanizing KettlesWhat are the special requirements for galvanizing kettles? . . . . . . . . . . . . . . . . . . . . . . . . . .8What factors determine kettle shape? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8How are pipe galvanizing kettles heated? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8What is the importance of kettle welds? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Zinc GradesWhat are the grades of zinc and how do they differ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10What grades of zinc are used in pipe galvanizing? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10Are there recommendations for sampling zinc ingots? . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Operating the Galvanizing BathWhat is the normal zinc bath operating temperature? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12How does heat input affect kettle life? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12Are insulation covers necessary? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Does aluminum affect the galvanizing bath? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13What are the sources of lead in the galvanizing bath? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14How are pipes submerged in the zinc bath? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

Sampling the Galvanizing BathWhy sample zinc in galvanizing baths? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16How should it be done? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

Dross and AshWhat is dross? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18What are sources of dross? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18How can dross be minimized? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

Galvanized CoatingHow is galvanized coating thickness controlled? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20Is it necessary to quench galvanized pipes? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

AppendicesAppendix A - Monitoring Sulphuric Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21Appendix B - Monitoring Flux Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27Appendix C - Sampling Zinc Ingots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31Appendix D - Monitoring Zinc in Galvanizing Baths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

Mass produced hot dip galvanized steel pipes and tubes are produced inplants set up specifically for galvanizing these products. While pipegalvanizing plants are usually associated with manufacturers of pipes andtubes, some tubular products are galvanized in batches in plants whichprovide custom galvanizing services. In addition, a few specialty plantsform and galvanize small diameter pipe continuously and cut it tocommercial lengths after galvanizing. In that case, only the outsidesurface of pipe and small diameter tube can be galvanized. Regardlessof galvanizing method used, processing variables within each plant needto be monitored and controlled to ensure that product quality andoperating efficiency are maintained at the highest levels.

What is pipe galvanizing?

Hot dip galvanizing is a universally accepted finishing operation used toprotect steel products from corrosion. Large scale galvanizing of pipe is asemi-continuous operation which uses special machinery to transportchemically cleaned pipe through a zinc bath, as shown schematically inFigure 1 (page 5). Galvanized products are automatically withdrawnfrom the zinc bath and immediately treated to remove molten zinc inexcess of coating specifications. Excess zinc is removed from the outsidesurface by “wiping” with compressed air, and from the inside by“blowing” with superheated steam.

While pipe galvanizing equipment has evolved and processes havechanged over the years, the underlying objective remains the same -clean the steel and metallurgically bond a zinc coating of specifiedthickness to the pipe surfaces (inside and outside).

4

The Process



What are the basic steps for galvanizing pipe?

Pipe galvanizing is made up of three steps: cleaning, fluxing, andgalvanizing. Each step must be completed effectively and efficiently toensure galvanized coatings are of the highest quality.

Steel can be galvanized only when its surface is chemically clean. Inpractical terms, this implies that the pipe surfaces are clear ofcontaminants such as oil and grease used during production, and free ofsteel-making mill scale and rust. Oil and grease are readily removed incaustic solutions which usually form part of the cleaning process.Pickling in acid is the most widely used method of removing millscaleand oxides from steel pipe. The choice of cleaning acid is largelydetermined by its availability.

5

The Process



FLUXRINSE

PICKLERINSE

DEGREASE

AIR

AIR

STEAM

OUTSIDE WIPE

INSIDE BLOW

GALVANIZING

Figure 1

Which acid should be used?

Both hydrochloric acid and sulphuric acid are effective for removingmillscale and rust from steel. While the choice of acid will undoubtedlybe influenced by economics and local availability, the opportunities todispose of used acid and access to effective methods of recycling usedacid are becoming increasingly important.

How can pickling acid be controlled?

Pickling rate is a primary requirement of pipe galvanizers as it has to becompleted relatively quickly and keep pace with the galvanizingproduction rate. Pickling rate is a function of acid concentration,temperature, and iron content whether hydrochloric acid or sulphuricacid is used. In practice, acid concentration and temperature should bemonitored and controlled to maintain efficient and effective picklingconditions. It is also critical to monitor iron content and either dispose ofthe acid when iron content reaches an operating maximum or removethe iron salts and regenerate the acid. Tests for monitoring sulphuric acidfor pickling are in Appendix A.

Most pipe galvanizers have favoured using sulphuric acid because itspickling rate is fast and can be matched to the galvanizing productionrate. Sulphuric acid is also relatively easy to regenerate.

6

Surface Preparation - Pickling



Why is flux used and how are pipes fluxed?

The galvanizing reaction will only occur effectively when the surfaces ofpipes are chemically clean. In practice, this means that the surface isfree of mill scale and rust. Pickling effectively cleans steel surfaces but afreshly pickled steel surface reoxidizes if left unprotected. These oxidesare readily dissolved by dipping in an aqueous solution of zincammonium chloride - the flux widely used by pipe galvanizers. Zincammonium chloride serves two purposes: it prevents additional oxidefrom forming on the surfaces of pickled pipes, and helps molten zincreact with these surfaces.

The concentration of the flux solution should be monitored and controlledto provide optimum results. Experience shows that fluxed pipes shouldbe dry before they enter the zinc bath otherwise some areas may fail toreact with the zinc and leave areas ungalvanized. These defects areknown as “black spots” in the industry. The use of a heated flux solutionwill help reduce the time needed to dry fluxed pipes.

Flux concentration should be adjusted according to ambient humidityand the time delay between fluxing and galvanizing. Immersion of pipesin the flux solution should be kept to a practical minimum to limit build-up of iron salts in the flux tank. Carry-over of iron salts with flux on thepipes will increase dross production and waste zinc.

How can flux solution be controlled?

It is necessary to control several key properties of the flux solution tomaintain fluxing at an effective level and to keep process economics inline. The following properties should be monitored and controlled:

• Flux concentration• Flux temperature• Iron concentration• Sulphate concentration

Tests for monitoring flux solution and control methods are described inAppendix B.

7

Surface Preparation - Fluxing



What are the special requirements for galvanizing kettles?

The galvanizing kettle and contained zinc represent the single mostexpensive part of a galvanizing plant. Unexpected kettle failure (leakage)in service will result in production stoppages and additional costs due toloss of zinc and damage to burners and furnace components. As such,the kettle should be designed and fabricated by experts in this field.

Kettles are commonly made of steel. Ceramic kettles, such as those usedin some other galvanizing operations, are typically unsuitable forgalvanizing pipe. Kettle fabricators can provide advice on sizing anddesign of the kettle, and can help select the correct grade and quality ofsteel for its construction. These fabricators also have equipment forforming heavy steel plates used for kettles and are familiar with thecorrect welding practices and specifications.

What factors determine kettle shape?

The galvanizing bath serves two purposes: 1) to heat the steel pipes togalvanizing temperature during immersion in the bath, and 2) to carryout the galvanizing reaction to produce an adherent zinc coating. Thekettle dimensions are largely determined by the maximum productionrate expected. Specifically, the kettle length is largely determined byproduction rate and maximum length of pipes processed. The width willbe influenced by the type of handling equipment used to transport pipesthrough the zinc bath. Kettle depth is mainly determined by the heatingmethod and overall thermal requirements. Pipe galvanizing plants aretypically high production units, so thermal requirements are often large.

How are pipe galvanizing kettles heated?

In general, galvanizing baths can be heated by gas, oil, or electricity butenergy choice will be influenced by local availability and economics.Energy requirements include heat to melt the zinc and keep it molten,heat to balance thermal losses due to radiation, convection, andconduction, and heat taken out of the kettle by the galvanized product.All heat required for the process has to be added to the galvanizing bath.Supplying heat through the kettle sidewalls is by far the most widely usedmethod but is limited to heating steel kettles.

Oil and gas are the favoured energy sources for pipe galvanizing. Oil

8

Galvanizing Kettles



heating allows high production rates but usually gives shorter kettle life.Electrically heated galvanizing baths are not normally used forgalvanizing pipe.

Oil and gas burners are normally located along the kettle sidewalls butcan be set at the corners. Regardless of which type of heating system isused, it is critical to prevent forming "hot spots" as they shorten kettle lifeand reduce thermal efficiency. Good furnace design, coupled with gasor oil-burning flat flame burners placed along the side walls, helps keeplocal overheating to a minimum.

Some kettles are heated by a single burner set in a combustion chamberlocated well away from the kettle. In this arrangement, combustiongases are circulated around the kettle. Correctly designed, this set-upprovides adequate heat input and relatively long kettle life.

High velocity burners, a more recently introduced method for heatinggalvanizing baths, are being used more widely. Placed at kettle corners,high velocity burners are designed so combustion gases pass along thelength of the sidewalls. The burners are programmed to "fire" at slightlydifferent times and for relatively short times so the flow of combustiongases is periodically reversed. This practice eliminates the danger offorming "hot spots".

What is the importance of kettle welds?

The galvanizing kettle and contained zinc are major capital items in pipegalvanizing plants. Furthermore, the kettle is indispensable since itsfailure stops production. Design and fabrication of galvanizing kettlesshould be entrusted only to experts since kettle integrity is of paramountimportance.

Steel kettles are made up of at least three plates welded to form a box.Vertical corners are normally radiused but sidewall-to-baseplate joins aretypically square. Welds are a potential source of weakness especially ifsubjected to overheating and erosion by molten zinc.

Where practical, kettle designers locate welds away from directly heatedareas and try to place vertical welds at the ends. Welds should be of thehighest standard and checked for complete penetration and freedomfrom porosity. Welds should be faced with low reactivity metal tominimize erosion in service.

9

Galvanizing Kettles



What are the grades of zinc and how do they differ?

There are three commercial grades of slab zinc: Special High Grade(SHG), High Grade (HG), and Prime Western (PW) grade. While thethree grades of zinc differ mainly by purity, PW grade zinc also has leadspecified as a range. Chemical requirements of slab zinc specified byASTM B6 (Standard Specification for Zinc) are shown in Table 1 below:

What grades of zinc are used in pipe galvanizing?

Commercial pipe galvanizing specifications define the minimumpermissible grade of zinc. Traditionally, pipe galvanizers have used PWgrade zinc, but more recently the trend is to use SHG or HG. This trendis due to environmental regulations associated with lead in potable water.

Are there recommendations for sampling zinc ingots?

Sampling of zinc used in pipe galvanizing plants is rarely necessary sincezinc producers provide chemical certification for each lot. However, pipegalvanizers may be required to sample and assay the zinc ingots tosatisfy quality control and assurance programs. Sampling of zinc ingotsmight also be required in rare instances of dispute resolution.

10

Zinc Grades



TABLE 1ASTM B6 Slab Zinc Chemical Requirements (in Wt%)

Element SHG Zn HG Zn PW Zn

Lead (Pb) 0.003 max. 0.03 max. 0.5 - 1.4

Iron (Fe) 0.003 max. 0.02 max. 0.05 max.

Cadmium (Cd) 0.003 max. 0.02 max. 0.20 max.

Aluminum (Al) 0.002 max. 0.01 max. 0.01 max.

Copper (Cu) 0.002 max. - 0.20 max.

Tin (Sn) 0.001 max. - -

Total non-zinc 0.010 max. 0.10 max. 2.0 max.

Zinc (by difference) 99.990 min. 99.90 min. 98.0 min.

Most pipe galvanizers can only use 25 kg zinc slabs for bath additionsbecause machinery used to transport pipes through the galvanizing bathprevents the use of larger blocks. However, larger one-ton blocks (jumboingots) of zinc produced commercially for other processes can be usedfor bath additions when galvanizing kettles are suitably modified.Materials handling savings are the main benefit of using jumbos.

It is feasible to sample zinc slabs and small ingots to check chemicalanalysis. Detailed methods of sampling slab zinc are described in ASTMB6. Procedures for sampling zinc slabs by drilling and sawing aredescribed in Appendix C.

Sampling of zinc jumbos is not recommended since it is difficult to getrepresentative samples. Disputes concerning the composition of jumbosshould be resolved by referring to producer reserve samples. These aresamples taken by the zinc producer to certify each production lot.

Figure 2

11

Zinc Grades



What is the normal zinc bath operating temperature?

The galvanizing reaction takes place relatively quickly within thetemperature range of 440°C to 470°C (825°F to 878°F). Operatingwithin this temperature range usually provides sufficient thermal back-up to maintain design production levels.

A nominal bath temperature of 450°C (840°F) is typical. The minimumand maximum operating temperatures are set by practical requirements.As zinc solidifies close to 419°C (786°F), the minimum operating bathtemperature must be maintained somewhat higher with a suitablethermal buffer. Steel galvanizing baths are relatively resistant to attackby molten zinc in the temperature range of 440ºC (825°F) to 470°C(880°F) but are severely attacked at higher temperatures (see Figure 3).Furthermore, galvanizing at the high end of the operating range cancause the zinc coatings to become fully alloyed, making them brittle andsubject to spalling. This tendency to form alloyed coatings is particularlytroublesome with steels commonly referred to as "reactive".

Figure 3(Source: American Galvanizers Association - Galvanizing Process Seminar)

How does heat input affect kettle life?

Steel used for galvanizing kettles reacts relatively slowly with zinc withinthe typical operating temperature range (see Figure 3) but dissolvesmuch faster at higher temperatures. Therefore, kettle life is shortened byoperating at higher galvanizing temperatures.

12

Operating the Galvanizing Bath



Relative Loss of Kettle Steel vs Zinc Temperature

05

1015202530354045

420 430 440 450 460 470 480 490 500 510 520

Zinc Temperature, ºC

Rel

ativ

e L

oss

of

Ste

el

Galvanizing Range

The service life of steel kettles heated through their walls is alsoinfluenced by the actual heat input. Projected kettle life decreases rapidlyas heat input increases (see Figure 4 - which shows data calculated for40 mm thick walls). In addition, local overheating causes severe erosionof the kettle steel and can result in kettle failure and subsequent loss ofzinc.

Figure 4

Burner failure and refractory damage can cause local overheating.Furnace design and burner location should aim for even heat distributionand limit heat input. Thermal requirements which are mainlydetermined by throughput and thermal efficiency (losses of heat from thesurface of the bath and conductivity to kettle foundations) also impactkettle design, especially bath depth.

Are insulation covers necessary?

Heat loss from unprotected surfaces of pipe galvanizing baths occurs byradiation and convection. Typical practice is to operate with a layer ofmolten flux on a large part of the zinc bath. Consequently heat loss isconsiderably lower than when the zinc bath is uncovered.

Pipe galvanizers find that a layer of molten flux also helps reduce coatingdefects.

Does aluminum affect the galvanizing bath?

Pipe galvanizers can add a relatively small amount of aluminum to thezinc bath to control oxidation and cut down ash formation. This practicealso reduces the risk of ash contaminating the galvanized coating.

13




Kettle Life vs Heat Input

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

12 24 36 48 60 72

Kettle Life, months

Hea

t In

pu

t,

kJ/m

2-h

r

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

Heat In

pu

t, Btu

/ft 2-hr

Some pipe galvanizers claim that a small concentration of aluminum inthe zinc bath also helps maintain a bright finish on galvanized pipe.However, galvanizers should be aware that aluminum readily reacts withchloride-based fluxes used in the process and reduces their effectiveness.Excessive aluminum in the bath can prevent some areas of the pipe frombeing galvanized. Ungalvanized areas are called “bare spots” or “blackspots” in the trade.

Experience shows that about 0.005% aluminum in the zinc bath issufficient to provide the benefits mentioned without affecting fluxperformance. The aluminum concentration in the galvanizing bath iscontrolled by making appropriate additions of zinc-aluminum masteralloys, referred to as brightener alloys. They typically contain 5 or 10%aluminum.

What are the sources of lead in the galvanizing bath?

There are two possible sources of lead in pipe galvanizing baths:

1) lead in the zinc additions (e.g. PW zinc), and2) primary lead added to the zinc bath.

Historically, pipe galvanizers used Prime Western grade zinc for bathadditions. When PW grade zinc is used, lead in excess of its solubilityin molten zinc will precipitate and collect at the bottom of the kettle. Thiscondition can be helpful since dross will float on the layer of lead andmake it easier to remove. Lead introduced with PW grade zinc can bein the range 0.5 to 1.4% (see Table 1). Lead levels measured inoperating baths are typically 1.2 to 1.4% (i.e. at saturation level).

The second source of lead is in cases where lead pigs are used whencommissioning new galvanizing baths. Lead helps protect the bottomof the bath and, as noted above, raises dross above the kettle floor plateallowing for easier dross removal.

14




How are pipes submerged in the zinc bath?

Chemically cleaned and fluxed pipes enter the zinc bath along the sideof the kettle. There are two widely used designs of pipe galvanizingmachinery. The main difference between them is the way pipes aresubmerged in the zinc bath. The pipes should be pushed into the zincbath at a slight angle to allow air and steam to escape freely. Besidesbeing an important safety feature, putting pipes into the kettle at an angleoften improves the quality of the galvanized coating on the inside ofpipes. Typically, several pipes are allowed to roll from a loading table intothe bath at any one time, often with manual assistance. Once in thebath, a mechanism lowers the pipes at a pre-determined rate. Thismechanism can be a pusher-rod type or a notched wheel. After residingin the bath for the prescribed time, the pipes are individually retrievedfrom the molten zinc.

Pipes are immersed in the molten zinc long enough to raise theirtemperature to above 440°C (824°F), at which point galvanizing occursat a reasonable rate. The desired coating weight will also be a factor.Immersion time, typically 2 to 4 minutes, is governed automatically bycontrolling the speed of the pipe galvanizing machine.

15




TABLE 1ASTM B6 Slab Zinc Chemical Requirements (in Wt%)

Element SHG Zn HG Zn PW Zn

Lead (Pb) 0.003 max. 0.03 max. 0.5 - 1.4

Iron (Fe) 0.003 max. 0.02 max. 0.05 max.

Cadmium (Cd) 0.003 max. 0.02 max. 0.20 max.

Aluminum (Al) 0.002 max. 0.01 max. 0.01 max.

Copper (Cu) 0.002 max. - 0.20 max.

Tin (Sn) 0.001 max. - -

Total non-zinc 0.010 max. 0.10 max. 2.0 max.

Zinc (by difference) 99.990 min. 99.90 min. 98.0 min.

Why sample zinc in galvanizing baths?

Sampling zinc in pipe galvanizing baths may be required for one of thefollowing reasons:

• To satisfy quality assurance programs• To meet customer contract requirements• To monitor the zinc bath• To help resolve galvanizing problems

In general, samples are analyzed to determine the levels of specificelements in the bath. For example, aluminum and lead are typicallyincluded. In addition, the levels of any other purposeful bath additions,such as nickel or bismuth (which can affect the coating appearance orthickness), should be known in order to control the process. Occasionally,copper and tin may be of interest, as these impurities can affect thecorrosion resistance of the coating.

How should it be done?

While in-process sampling of galvanizing baths is feasible and providesreliable results, differences in plant set-up and equipment arrangementwill influence which sampling practices can be used. Access forsampling pipe galvanizing baths is made difficult by the presence ofmachinery used to move pipes through the zinc bath, as well as ductingused to remove fumes from the galvanizing bath area. A blanket ofmolten flux on the surface of the bath further limits the opportunities fortaking bath samples from pipe galvanizing baths. Considering theselimitations, sampling where the galvanized pipes are withdrawn from thezinc bath will likely be the most convenient and practical location.

16

Sampling the Galvanizing Bath



Zinc samples should be taken from the galvanizing bath during normaloperation. Samples should not be taken immediately following additionsof zinc ingots or master alloys, unless special studies justify sampling atsuch times.

Sampling molten zinc in pipe galvanizing baths is done in two parts:

1) Collecting sufficient molten metal to form a representative sample,and

2) Solidifying the molten sample in a shape suitable for analytical testing.

Samples are best formed in permanent molds because chill-castingminimizes segregation of impurities and alloys within them. Pin-shapedsamples can be analyzed by Atomic Absorption (AA) and InductivelyCoupled Plasma (ICP) spectrometric techniques. Chill-cast disks (shownbelow) can be analyzed directly by Optical Emission Spectrometry(OES), as well as by the other two techniques. Details are provided inAppendix D.

17

Sampling the Galvanizing Bath



Figure 5

Illustrations of vertical disk and horizontal disk samples showing

preferred target areas for OES as per ASTM E634.

What is dross?

Dross is an unavoidable product of the galvanizing process. It is a zinc-rich compound (FeZn13) formed when iron in the galvanizing bathexceeds its solubility in liquid zinc. It is this excess iron that reacts withzinc and precipitates as iron-zinc particles.

Most dross settles to the bottom of the bath and is removed before itinterferes with the pipes passing through the zinc bath. While there is amarket for dross, there is a direct cost resulting from the difference inprice between zinc and dross and an indirect cost due to the labourrequired to remove the dross.

What are sources of dross?

Dross is a by-product of the galvanizing reaction and also forms as zincreacts with the steel kettle. Molten zinc also chemically reduces iron saltsto iron which forms additional dross.

There are four primary sources of dross:

1) Pickle salts (iron salts formed by the reaction of pickle acid and steel),

2) Flux salts (iron salts formed by the reaction between flux and steel),3) Iron-zinc interaction during galvanizing of steel (the direct reaction

between steel pipes and molten zinc), and4) Iron-zinc alloy (formed from the reaction of molten zinc with the

steel kettle)

18

Dross and Ash



How can dross be minimized?

Although the formation of dross can never be prevented, the amountformed can be kept to an acceptable level by monitoring and controllingthe galvanizing process. The following practices can help keep drossproduction to a minimum:

• Keep pickling to a practical minimum• Use acid inhibitors to help avoid excessive pickling• Control iron concentration in pickle acid• Avoid drying work between pickling, rinsing and fluxing• Rinse the pipe in water after pickling to avoid contamination of the

flux• Control iron concentration in flux solution• Minimize immersion time in flux• Galvanize at the lowest practical temperature• Minimize immersion time in zinc• Avoid wide fluctuations in zinc bath temperature

Practices that reduce dross formation will also lower operating costs.Examples include:

• Less chemicals required when flux composition is controlled• Zinc savings achieved when dross production is kept to a

minimum• Energy savings achieved when the kettle is operated at a lower

temperature• Kettle life will be extended• Labour savings result from less dross build-up (less frequent

removal)

19

Dross and Ash



How is galvanized coating thickness controlled?

Excess zinc dragged out of the galvanizing bath on galvanized pipes isremoved automatically. This is done by passing each pipe through aperforated ring blowing superheated air onto the outside of each pipeupon removal from the galvanizing bath. Pressurized steam is used toremove excess zinc from the inside of each pipe.

Is it necessary to quench galvanized pipes?

Galvanized pipes can be passed through a quench tank to prevent thezinc-iron alloy phase in the coating from extending throughout thethickness of the galvanized coating. The zinc-iron alloy continues todevelop until the temperature of the galvanized pipe is well below thegalvanizing temperature. Fully alloyed coatings can be brittle and, if toothick, are liable to spall.

Quench tanks usually contain warm water, which also helps avoiddistortion during cooling, but may also contain passivation chemicalssuch as chromate. Such passivation treatments minimize formation ofstorage stain (white rust) on freshly galvanized products.

20

Galvanized Coating



Sulphuric acid, typically used as a 10% by weight solution, is widelyused for pickling pipes and tubes to be hot dip galvanized. The picklingrate depends on several factors:

• thickness and type of scale• acid temperature• acid concentration• iron concentration in acid• use of inhibitors• acid agitation

Acid TemperatureThe usual operating temperature for sulphuric acid is in the range 65 -85°C (150 - 185°F). Pickling times increase as acid temperatureincreases. Sulphuric acid can be used for pickling down toapproximately 60°C (140°F) but the pickling rate may not keep up withthe galvanizing production rate. Pickling at temperatures up to 90°C(194°F) is also possible but steam and mist may create hazardousworking conditions unless pickling tanks are adequately vented.

21

Appendix A - Monitoring Sulphuric Acid



Acid ConcentrationSulphuric acid pickles at rates suitable for most pipe galvanizingoperations when used within the 8 to 12% acid concentration range.Pickling rates are relatively high within this concentration range. Theeffect of acid concentration and acid temperature on pickling rate isshown below.

Figure 6

(Source: American Galvanizers Association - Galvanizing Process Seminar)

Iron ConcentrationThe iron concentration of pickle acid increases with use and causespickling rate to decrease. While pickling rate can be increased byadditions of fresh acid, pickling rate decreases progressively until it isunacceptably low at about 7% iron. At this point, the pickle acid shouldbe replaced or treated to recover remaining free acid.

Use of InhibitorsInhibitors retard the attack of acid on the surfaces of cleaned steel andlimit the possibility of over pickling. Inhibitors can lower the rate of attackof the acid on chemically clean steel by up to 95% and reduce theamount of hydrogen evolved by the same amount. Some inhibitors forma blanket of foam on the surface of pickling baths and cut down theemission of acid mist. Used effectively, inhibitors reduce costs and loweracid consumption. Other benefits include improved working conditionsand safety around pickling tanks.

22




Typical Pickling Time vs Sulphuric Acid Temperature

0

10

20

30

40

50

60

70

80

2 4 6 8 10 12

Acid Concentration, wt%

Pic

klin

g T

ime

(min

ute

s) Acid Temperature, 80ºCAcid Temperature, 70ºCAcid Temperature, 60ºC

The addition of an acid-stable wetting agent to pickle acid will likelyimprove the rinsing of pickle acid from cleaned work surfaces.

Note: Chemical suppliers should be consulted for information onsuitable wetting agents and inhibitors. Inhibitors should be usedsparingly because excess will significantly decrease pickling rate.

AgitationPickling rate can be increased by agitating the pickle acid. Pipes andtubes should also be periodically raised and lowered within the picklingtank to move acid inside the pipes. In the absence of agitation, pickleacid becomes stagnant, acid concentration decreases locally and ironconcentration increases. The overall effect is a reduction in pickling rateand effectiveness.

Maintaining the Acid BathTo operate pickling baths efficiently and economically, it is necessary toknow when to add fresh acid and when to treat or replace ineffectivepickle acid. Monitoring of pickle acid involves measuring iron and free-acid contents and temperature. This data should be determinedregularly and at reasonably frequent intervals. It is also recommendedto monitor and record the tonnage of work pickled, additions of freshacid, and inhibitors, if used.

Maintaining the free acid content of the pickle solution is done by makingregular additions of small amounts of fresh acid. The pickle bath shouldbe discarded when the iron concentration reaches approximately 7%and the free acid content has been worked down to a minimum practicallevel. Each 2% of free sulphuric acid consumed in pickling will raise thesoluble iron in the pickle bath by about 1%. Therefore, when the ironconcentration reaches 4 to 5%, additions of sulphuric acid should bediscontinued allowing the free acid to work down to the point wherepickling rate becomes too slow. As the acidity is being worked down,the pickling rate should be maintained by increasing the acidtemperature.

23




24




Measuring Density (S.G.) of Pickle Acid The specific gravity (S.G.) of pickle acid can be used to estimate ironconcentration. It can be measured directly using a hydrometer for liquidsdenser than water. Alternatively, density can be measured in degreesBaume (ºBé) and converted to S.G. using the following relationships:

S.G. =145

°Bé = 145 - (145 ÷ S.G.)145 - °Bé

Equipment:• Thermometer• Hydrometer of suitable range• Graduated Cylinder

Method:1. Collect sample of pickle acid in graduated cylinder - fill to within

25 mm - 30 mm of top.2. Adjust solution temperature to 15.5 ± 0.3°C (60 ± 1.8°F).3. Warm hydrometer to the same temperature as sample solution; dry

hydrometer.4. Immerse the hydrometer in the sample of pickle acid to a point

slightly below the equilibrium level and allow it to come to restfloating an equal distance from the sides of the graduated cylinder.

5. Read the hydrometer at the point where the plane of the liquidsurface cuts the hydrometer stem. Check the solution temperatureand repeat the test if the temperature has not remained within the15.5 ± 0.3 C° (60 ± 1.8°F) range.

6. Record either S.G. or °Baume (°Bé) to the nearest 0.05°Bé reading.

For control purposes, measurements can be made at highertemperatures and temperature-adjusted as follows:

Add 1°Bé if temperature is 38 - 60°C (100 - 140°F)Add 2°Bé if temperature is 61 - 82°C (141 - 180°F)




25

Measuring Free Acid in Pickle Acid Equipment:

• Two - 250 mL beakers• 250 mL conical flask • Pipette (2 mL)• Supply of 0.408N sodium carbonate (21.6 g/L) solution• Hydrometer to read S.G. in range 1.0 - 1.3• Small glass funnel and filter papers• 50 mL burette

Method:1. Collect a sample of pickle acid (in a 250 mL beaker) and filter it into

another 250 mL beaker. 2. Pipette 2 mL of the filtered sample to a 250 mL conical flask to which

100 mL distilled water has been added. Mix diluted sample, add 3or 4 drops of Methyl Orange indicator, and mix again.

3. Titrate the diluted sample with 0.408N sodium carbonate solutionfrom the burette, agitating the flask continuously. Record the mL of0.408N sodium carbonate solution required to cause the colour ofthe indicator to change from red to yellow.

4. Read the acid concentration in g/100 mL directly as the number ofmL 0.408N sodium carbonate used.

Weight % sulphuric acid =g/100 mL sulphuric acid

S.G

26




Measuring Iron Content in Sulphuric Acid Pickle While there are analytical tests to measure the iron content in pickle acid,use of the S.G. - Sulphuric acid - Iron nomogram below will provide datasufficiently accurate for control purposes. The nomogram graphicallyrelates g/L iron in the pickle acid to acid S.G. and sulphuric acid strength(g/L). It is necessary to measure only S.G. and acid strength to determineiron content.

Examples: Line A-A; S.G. 1.25; Sulphuric Acid Conc. 50; Iron 85Line B-B; S.G. 1.15; Sulphuric Acid Conc. 90; Iron 35

Iron content can also be calculated using the following equation:

Iron g/L = (393.458333 x S.G.) - (0.25025 x g/L H2SO4) - 393.7375

Sulphuric Acid NomogramDensity, S.G. Sulphuric Acid Conc.

g/L1.30

1.25

1.20

1.15

1.10

1.05

1.00

0

50

100

150

200

250

300

1009080706050403020100

Iron Content, g/L

A-A

B-B

27

Appendix B - Monitoring Flux Solution



Measuring Density of Flux SolutionDensity is measured in °Bé with a hydrometer that has a range for liquidsdenser than water.

Equipment:• Thermometer• Graduated Cylinder • Hydrometer of suitable range

Method:1. Collect a sample of flux solution and fill the graduate cylinder to

within 25mm - 30mm of the top.2. Adjust solution temperature to 15.5 ± 0.3°C (60 ± 1.8°F).3. Warm hydrometer to temperature of sample; dry hydrometer.4. Immerse the hydrometer in the sample of flux solution to a point

slightly below the equilibrium level and allow it to come to restfloating an equal distance from the sides of the graduate cylinder.

5. Read the hydrometer at the point where the plane of the liquidsurface cuts the hydrometer stem. Recheck the solutiontemperature and repeat the test if the temperature has not remainedwithin the 15.5 ± 0.3°C (60 ± 1.8°F) range.

6. Record ºBé at 15.5ºC to the nearest 0.05ºBé reading.

For control purposes, measurements can be made at higher sampletemperatures and adjusted as follows:Add 1°Bé if temperature is 38 - 60°C (100 - 140°F)Add 2°Bé if temperature is 61 - 82°C (141 - 180°F)

S.G. =145

°Bé = 145 - (145 ÷ S.G.)145 - °Bé

Monitoring Iron in Flux Solution Monitoring iron in flux solution involves two tests:

1) Determining if iron is present in the flux solution, and 2) Measuring the iron concentration.

1) Determining if iron is present in the flux solution1. Collect about 100 mL of flux solution from the flux bath and

add 5 mL of 35% hydrogen peroxide (H2O2) diluted 5:1 withdistilled water.

2. Mix and adjust to pH 4.5 - 5.0 with ammonium hydroxide. Ironis present if the solution turns a reddish-colour. If present, ironcontent in the flux solution should be determined.

2) Measuring the Iron ConcentrationPotassium Permanganate Method:Iron in the flux solution is in the ferrous (Fe2+) state. Titration withKMnO4 is a direct method of determining iron content in fluxsolution. The highly coloured permanganate defines the titration endpoint. Oxidation of Fe2+ to Fe3+ by the permanganate ion (MnO4

-)is the basis for this test: (Fe2+ + MnO4 –›Fe3+ + MnO2).

Reagents:Potassium permanganate (KMnO4) 0.100N solution; prepared bydissolving 3.1608 g of KMnO4 crystals in distilled water and diluting to1 L. (Hold the permanganate solution in a dark bottle and store for 24hours before using.) Zimmerman-Reinhardt solution; prepared by mixing 67 g manganoussulphate (MnSO4.4H2O) in 700 mL distilled water. Add 56 mL of 1.83S.G. sulphuric acid (H2SO4) reagent and 133 mL of phosphoric acid(85% meta-H3PO4). Dilute to 1 L.

Equipment:• 50 mL burette• 250 mL Erlenmeyer flasks• 10 mL pipette• 50 mL graduated rubber bulb

28




29




Method:1. Pipette exactly 10 mL of sample solution into 250 mL Erlenmeyer

flask using rubber aspirator bulb and add about 25 mL of distilledwater.

2. Add 25 mL of Zimmerman-Reinhardt solution to flask.3. Titrate with 0.1N KMnO4 solution to the first pink colour that persists

for 20 seconds.4. Record mL of 0.1N KMnO4 required.5. Measure and record flux solution density and temperature.

Calculation:

% Iron =mL 0.1N KMnO4 used x 0.005585 x 100

mL sample x sample density(g/mL)

Controlling Iron Concentration in Flux SolutionThe iron concentration in the flux solution should be maintained below0.5% because excess iron will form additional dross. Iron concentrationabove 0.5% is a sign of inadequate rinsing after pickling or highly acidicflux.

Filtering will remove insoluble iron from flux solution but soluble ironmust be treated chemically to precipitate it before filtering.

Treatment of flux solution with 35% hydrogen peroxide (H2O2) willprecipitate iron as ferric hydroxide which can be separated from the fluxby filtering. Alternatively, ferric oxide can be allowed to settle then beremoved by suction, or the purified flux solution can be pumped into aclean holding tank.

Procedure for controlling iron in flux solution:1. Cool flux solution.2. Add a predetermined volume of hydrogen peroxide to the flux tank.3. Adjust flux solution to pH 4.5 - 5.5 (ammonia can be used).

Note: pH control is important since iron may not be completelyprecipitated at pH <4.5 and zinc may be precipitated at pH >5.5.

30




Example:If flux tank volume = 25,000 LFlux density = 1.155(kg/L)Iron content = 1.3%Total weight of flux = tank volume (L) x density(kg/L)

= 25,000 x 1.155 = 28,875 kgWeight of iron in flux = flux weight (kg) x % iron

= 28,875 x 1.3/100=375 kgWeight of H2O2 = weight of iron (kg) x 1.3

= 375 x 1.3 = 487 kg

Determining Free Acid in Flux SolutionMethod:1. Collect a sample of flux solution in a 250 mL beaker.2. Transfer 20 mL of sample to a 250 mL conical flask containing 100

mL distilled water. Mix, add 3 or 4 drops of Methyl Orange indicatorsolution, and mix again.

3. Titrate with 0.408N sodium carbonate (21.6 g/L) to the indicatorcolour change.

Calculation:

Acid concentration, g/100 mL= mL 0.408N sodium carbonate solution used

10

Controlling Free Acid in Flux SolutionThe concentration of free acid in flux solution should not be allowed toexceed 2 g/L as it will dissolve iron from the material during fluxing. Ironsalts carried into the galvanizing bath will form dross and waste zinc.

Free acid in flux solution can be neutralized with ammonia or reactedwith slab zinc. Clean waste zinc from around the kettle can also be usedto neutralize the acid.

Note: Dross should not be used to neutralize free acid since it containsabout 3% iron.

31

Appendix C - Sampling Zinc Ingots



ASTM specification B6 (Standard Specification for Zinc) describesmethods of sampling zinc slabs. Methods of sampling slab zinc bydrilling and sawing are described below:

Sampling Slabs by DrillingHand-held power drills and bench-mounted drills are suitable for takingsamples from 25 kg slabs. Drilling is also suited to sampling small ingotsand bars. ASTM B6 recommends the drill used be one twisted from flatstock. The diameter of the drill should be 12.7 mm for sampling SHGzinc and 7.9 mm diameter for sampling HG and PW grades of zinc andzinc alloys.

Drilling conditions should be adjusted to give drillings approximately0.25 to 0.50 mm thick. The drillings should be broken or cut with cleanshears into pieces less than 13 mm in length and mixed thoroughly.

Two holes should be drilled, preferably from the bottom or brand side ofeach slab at two points located along one diagonal of the slab so thateach point is halfway between the centre and one extremity of thediagonal. Each hole should be drilled completely through the slab.Drilling should be carried out without the use of a coolant or lubricant.

Sampling Slabs by SawingPower hacksaws are best used for sampling zinc slabs of 25 kg nominalweight. ASTM B6 recommends using a heat treated high-speed steelsaw blade. Two cuts should be made completely through the slab beingsampled from one long-side to the other. Each cut shall beapproximately halfway between the centre and each end. Sawingshould be carried out without the use of a coolant or lubricant.

Note: Sawing SHG zinc slabs to obtain samples is not recommendedbecause it is difficult to remove final traces of iron accidentally pickedup by the sawings during cutting.

32

Appendix D - Monitoring Zinc in Galvanizing Baths



Key parameters related to the galvanizing bath should be monitoredregularly to provide a reliable record of operating conditions.Recommended measurements include the following:

• Zinc bath temperature (measure continuously using thermocouples)• Zinc bath chemical analysis (on a daily or weekly basis, as required)• Grade and weight of zinc used (when materials are added)• Weight of dross removed and frequency of removal (after each

drossing)

The types of bath samples and how each type of sample can beanalyzed are described below. It is recommended that samples ofmolten zinc taken from galvanizing baths be chill-cast to minimizesegregation of impurities and alloyed elements within the sample.

Samples for determining the zinc composition are readily chill-cast inpermanent molds as pin or disk-shaped castings. ASTM E 634(Standard Practice for Sampling of Zinc and Zinc Alloys for OpticalEmission Spectrometric Analysis) provides guidelines for producing thesetypes of samples. The assaying method to be used will determine whichtype of sample is preferred and govern how the sample has to beprepared for analysis.

Pins: Samples chill-cast as pins are best analyzed by AA or ICP.However, pins (or smaller samples taken from them) can be re-melted, homogenized, cast as disks and analyzed by OES.While this procedure is permitted, it is not recommended sinceit takes extra time and may introduce impurities to the sample.

Disks: Samples formed as disks are designed for assaying by OpticalEmission Spectrometry (OES). Disks can be assayed directlyfollowing relatively straightforward, yet important surfacepreparation. This involves removing the face material by meansof a lathe or milling machine (rather than a grinding belt).Smaller samples can also be taken from sample disks by drilling,turning, or milling and assayed by AA or ICP.

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