Dross Formation in the Galvanizing Kettle

By Bernardo Duran

Goal of Presentation

Discuss why dross forms in the galvanizing kettle and methods to

decrease its formation.

Overview

Definition of dross

Why dross formation matters

How dross forms

How to reduce dross formation and its negative effects on the finished product

Definition of Dross

Byproduct of the galvanizing process which consists of loose iron particles (iron salts) that have metallurgically reacted with zinc in the galvanizing kettle.

Dross can contain more than 94% zinc (6% iron).

Why Dross Formation Matters

Aesthetic concerns from dross pimples.Steel can be rejected for gross dross inclusions.Dross formation accounts for approximately 15% of the zinc usage in the galvanizing process.Dross formation can be reduced.

How Dross Forms

Free iron particles in the galvanizing kettle metallurgically react with zinc to create dross particles rather than the zinc reacting with the steel to create a galvanized coating.

Types of Dross

Floating Dross: free particles of dross that can float throughout the kettle and usually consist of long intermetallic spikes interwoven in clumps; hollow-like structures

Bottom Dross: dross particles that aggregate (settle) to the bottom of the galvanizing kettle

Floating Dross

Dross Pimples

Dross Pimples

Gross Dross Inclusion

Where the Free Iron Particles Come From

Iron salts formed by a reaction between pickling agents and steel

Iron salts formed by reaction between flux and steel

Zinc/iron alloys formed by a direct reaction between steel and molten zinc (loose iron particles on the steel)

Iron Salts from Pickling Agents

Pickling agents react with iron oxide and the steel to create iron salts.

Creates loose iron salts that can then be transferred to subsequent tanks if not rinsed properly.

Iron Salts from Reactions with Flux

Fluxing agents such as ammonium chloride and zinc chloride can form iron salts when reacting with the steel which can then be carried over to the galvanizing kettle.

Free Iron from Zinc/Steel Reaction

Loose particles of iron from the iron or steel that comes off while in the kettle can go on to create dross particles.

Reactive steels can develop excess zeta layer formation which can flake off crystals that create free iron particles.

New Galvanizing Kettles

Dross formation can be high in new kettles until a layer of intermetallic layers form on the kettle walls, which then usually inhibits further dross formation.

Reducing Dross Formation

Limiting formation of iron salts during pickling operationAdequately rinsing iron salts after pickling operationMonitoring iron levels in pickling and flux baths, and galvanizing kettleElemental additions to galvanizing kettleAvoiding large temperature swings in galvanizing kettle

Limiting Iron Salt Formation During Pickling

Inhibitors can be used which limit the attack of the acid on the steel while not affecting dissolving action of the iron oxide and mill scale on the steel

Monitor pickling time to ensure steel is not over pickled

Adequate Rinsing of Pickling Salts

Allow enough time for pickling salts to drain before transferring the steel to the rinse tank

Allow enough time in rinse tank

Implement a second rinse tank when possible/practical

Monitoring Iron Levels in Pickling and Flux Solutions

Frequently check iron levels and pH in pickling tank and rinse tanks

Frequently check iron levels in flux (should be no greater than 0.5%)

Aim for an iron level of about 0.1% in the galvanizing kettle (iron solubility will vary with kettle temperature)

Properly Maintain Flux Solutions

Dross formation can be higher in wet flux method than dry method

Agitation of the flux in the dry process can help to convert excess iron to sludge

Work with flux supplier to find optimal chemical levels and filtering strategies

Elemental Additions to the Galvanizing Kettle

Small additions of lead (1%) can reduce dross formation, but the industry is moving away from lead use in kettlesNickel additions (0.04 to 0.09%) can reduce floating dross (however, nickel decreases the solubility of iron and can increase bottom dross formation); (see iron solubility on next slide)Adding elements to kettle in smaller quantities more frequently is preferred over bigger quantities less frequently

Iron Solubility in Zinc and Zn-Ni (Courtesy of Teck)

Maintaining Consistent Temperatures in the

Galvanizing KettleIron solubility increases with increases in kettle temperature (see graph on next slide)

When temperature drops, the iron precipitates out of solution which is then available to form dross

Uppermost level of zinc can be cooler than lower levels of zinc and have lower iron solubility

Solubility of Iron in Zinc with Varying Temperatures

Removing Dross from Kettle

Bottom dross should be removed on a scheduled basis, whether by time or steel volume throughput

Nitrogen can be bubbled in zinc (ensure bottom dross is not disturbed) to float dross to the surface where it can then be scooped out

Conclusion

Reducing dross formation: Increases profits because less zinc is consumed in the form of dross

Makes for happier customers since there are less dross inclusions on the steel

Additional Resources

Galvanizing Note: Skimmings & Dross

Troubleshooting Guideline: Reducing Dross Pimples

AGA Resource Library

Your pickling and flux solution suppliers