A Novel Thermal Spray Coating for Galvanizing Rolls Ram K

Technovations International Inc [email protected] UniqueCoat Technologies, LLC [email protected]

A Novel Thermal Spray Coating for Galvanizing Rolls Ram K. Iyengar, Ph. D., Technovations International Inc.

Viatcheslav E. Baranovski, Ph. D., Uniquecoat Technologies, LLC. Introduction In continuous galvanizing, rolled steel strip is immersed in a bath of molten zinc/aluminum alloy. The steel strip is dipped in the molten bath through a series of rolls, which control the speed and tension of the strip and guide the steel strip through the molten metal bath.

Figure 1: Sink and Stabilizer rolls in a galvanizing line The rolls operating in the molten Zn-Al alloy are subjected to severe corrosive environment and require frequent change and repair. A typical galvanizing shop experiences downtimes on the order of every two weeks to change the rolls. It takes upwards of 3 days for a complete turn around to resume normal operation. This results in loss of production and profits. Poor quality of sink and stabilizer rolls can lead to defects in the strip due to:

• Roll not turning, • Sink roll groove marks appearing on the galvanized strip. • Dross build up on rolls.

Figure 2. Dross defects on a galvanizing roll


Dross The dross particles are formed in the V region where the strip enters the bath, contacts the sink roll and exits the pot. Dross buildup on 316L SS rolls is the result of two chemical reactions:

• Between steel and the galvanizing bath to produce a surface reaction layer. • Interaction of this layer with dross particles suspended in galvanizing bath.

Iron in the steel strip reacts with zinc and aluminum in the molten bath to produce dross as per the following chemical reaction: 2FeZn7 (Bottom Dross) +5Al Fe2Al5 (Top Dross) + 14Zn The dross is picked up by the steel strip and causes a defect. This is a major quality problem for some galvanizers. Thermal spray coating One approach currently being used to decrease dross pick-up on the strip is by using rolls coated with ceramic-metals (CERMETS) such as cemented carbides or borides. The coating processes are based on heating powdered materials to appropriate temperature and propelling them at supersonic velocity against the roll surfaces to produce coatings with desired properties. These processes include: high velocity oxygen fuel (HVOF), activated combustion high velocity air fuel (AC-HVAFTM is a trade mark of UniqueCoat® Technologies, LLC), plasma, arc etc. These processes operate at different temperatures with particle velocities reaching up to Mach 3.


Figure 3. Temperature and velocities in thermal spray coating processes AC-HVAF is a process in which a mixture of compressed air and fuel gas (propane, propylene, natural gas, or MAPP-gas) is combusted in a combustion chamber, generating high velocity jet that exhausts out of a nozzle. A hot wall of the ceramic permeable burner block activates the combustion process. Such design provides stable air-fuel combustion within very short chamber, allowing axial injection of spray particles through it. Powder particles are heated to targeted temperature and accelerated to velocity approaching that of the gaseous jet. The powder particles are plastically flattened on the surface and form a coating. This process provides coating with low porosity and high adhesion. Additionally, AC-HVAF coatings, unlike HVOF counterparts, practically do not oxidize spray powders. This creates perfect conditions for coating sintering during service under elevated temperature in zinc bath.

Figure 4. AC-HVAF thermal spray coating Surface coatings prevent wetting and dissolution of the roll material by the molten alloy. High erosion resistance against molten metal is the most important property for the thermal spray coating material. Thermal spray coating with carbides significantly decreases the attack rate of Zn, and provides excellent resistance against molten zinc. Double carbides of η-phase (Co3W3C and Co6W6C) in the WC/Co coating is found to be effective to improve the durability, where η-phase reduces free metal Co with high reactivity. The effect of the η-phase coating is limited to relatively lower temperatures below 890 °F.


Galvanizing lines with Al-45wt.%Zn alloy operate at temperature above 1150 °F. At these temperatures thermal spray coating with carbides are not effective. Therefore novel surface modifications are necessary to:

• Prevent wetting and adhesion of dross and molten alloys on the roll surface, • Minimize the dissolution of the roll substrate by the molten alloy, • Increase operating time and ensure defect free products.

Laboratory Tests Laboratory research has shown that boride based thermal spray coating meets the above requirements for temperatures above 900 ˚F. Immersion tests have demonstrated that the MoB based thermal spray coating has much higher durability in the molten Al and Al-Zn alloys than conventional thermal spray tungsten carbide coatings. No damage or dissolution was observed in the MoB based coating after long-term immersion in molten Al-45wt.%Zn alloy. On the other hand conventional cermet and ceramic coatings were damaged due to dissolution or crack generation. Coating with tungsten carbide showed no adhesion of the molten alloy on the surface up to 100 hours. Thereafter adhesion increased gradually and the dissolution of the substrate was observed at 209 hours. Tungsten carbide coatings with lower carbon content showed higher durability, but the molten alloy did adhere to the coatings suggesting that there was a reaction between the molten alloy and the coating material. The MoB based coating showed excellent durability without dissolution and mechanical damage even after 623 hours of immersion. EDX analysis showed very little diffusion of both Al and Zn into the MoB based coating. The crystal structure of the MoB based coating was characterized by double borides. There was no structural change when the coating was kept with prolonged contact in molten Zn-Al alloys. This results in the higher durability. MoB based cermet materials have excellent durability against the molten aluminum because of the matrix in MoB based coating is a Co-based alloy compared with the pure Co metal matrix in the tungsten carbide coatings. This inertness of the matrix may be the reason for the higher erosion resistance of the MoB based coating. In conclusion, MoB based thermal spray coating prevents wetting by molten alloy without being attacked in molten Al-45wt.%Zn alloy, and has excellent durability compared to the conventional carbide coating. Plant Trials Trials were carried out at two galvanizing plants with thermal spray coated rolls using tungsten carbide and the novel new material based on molybdenum boride. In the first trial, thermal spray tungsten carbide coated roll showed delamination of coating after use as seen in the photograph below.


Figure 5. Thermal spray coated roll showing delamination of tungsten carbide coating after use.

The companion roll that was thermal spray coated with molybdenum boride based coating showed better adhesion.

Figure 6. Molybdenum boride coated roll The molybdenum boride coated roll showed promise but did not give the desired roll life perhaps due to bearing failure. The thermal spray parameters and the coating structure were modified on the next trial using a smaller area of a spindle as shown below.

Figure 7. Showing a sketch of the area coated on the spindle.


This spindle was attached to the drive for the stabilizer roll and was removed along with the roll. The frozen alloy was easily removed with a sharp crow bar and the coating was inspected. Visual inspection showed that there was no reaction between the Zn-Al molten alloy and the coating. In the third trial, two more stabilizer rolls were coated for two different galvanizing plants. The surface of the roll from one plant is shown below.

Figure 8. Molybdenum boride coated roll before use The Molybdenum boride coating was intact after the roll was removed from the service. The frozen dross in the Zn-Al alloy is due to the solidification and not due to build up on the roll.

Figure 9. Molybdenum boride coated roll after removal from service. The frozen layer was easily removed.

Figure 10. Full view of the roll after removal of the frozen alloy


Figure 11. Non-driven end, center face and driven end after removal of the frozen alloy The second roll at another plant showed good performance as per the following communication from the Area Manager, Galvanize.: “I would just like to say your molybdenum alloy coating I used for my 60” coating pot stabilizer roll increased the campaign life by four weeks. Normally, for a standard blasted roll, it would be changed within three weeks. With Technovations, molybdenum alloy coating, I was able to use the same roll for seven weeks. Thank you for providing us a quality product that saved us around $10K.” The following photographs show the state of the roll after removal from service.

Figure 12. Roll after removal The roll was taken out of service to replace the bearings. There was no dross build up and the roll was generally not wetted by molten alloy in most of the roll surface as expected. The frozen dross in the alloy seen on some parts of the roll melted away when the roll was put back in the galvanizing pot after repair of the bearings.


Figure 13. Roll showing small areas of the frozen dross in alloy.

Figure 14. Non-wetted ends of the roll coated with molybdenum boride. The boride coated roll gave a life of seven weeks compared with the normal three weeks. Conclusion: Laboratory immersion tests had shown that MoB based coating performed better than the tungsten carbide coating. This is also demonstrated in several industrial scale trials in two continuous galvanizing plants. Technovations International Inc and UniqueCoat Technologies, LLC have developed a novel thermal spray coating with a molybdenum boride based powder. The coating is not wetted by the Zn-Al alloy and does not react with molten Zn-Al alloy even at high temperatures. There is no dross build up on the roll, which enables the reduction of defects on galvanized strip. The frozen alloy on the roll is easily removed and the roll can be used again to get longer roll life. Technovations International Inc ([email protected]) and UniqueCoat Technologies, LLC ([email protected]) invite galvanizing plants to demonstrate for themselves how this novel coating can benefit their operation and produce higher quality products. Contact: Ram K. Iyengar, Technovations International Inc., 5 Betsy Lane Littleton MA 01460, 978-952-2955; or Viatcheslav Baranovski, UniqueCoat Technologies, LLC, 1070 Merchants Lane, Oilville VA, 23129, 804-784-0997.

A Novel Thermal Spray Coating for Galvanizing Rolls Ram K

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