Overcoming Outgassing Issues During Painting or Powder Coating of Zinc-Plated Parts
BY ANDREW.GRUDA, CEF TECHNICAL MANAGER, GAITO INDUSTRIAL PLATERS, INC., CHICAGO, IL.
Zinc or zinc nickel alloy electroplating of steel or cast iron parts is one of several widely specified and used methods of enhancing overall performance and corrosion protection of painted and/or powder coated parts.
Combining an undercoat layer of electroplated zinc or zinc nickel alloy with its sacrificial protection mechanism and top coat layer of wet paint or powder coat insulating mechanism increases corrosion protection many times over versus individual finishes.
Electroplated zinc or zinc alloys, plus chromate conversion film typically applied as a post treatment, is able to provide good adhesion for paint or powder coating. In most cases, if chromate conversion film is already part of the plated finish and the time between plating and painting is done within a specified window, parts can be painted or powder coated without additional chemical pretreatment.
It is worth mentioning that subjecting such a plated and chromated parts to standard three- or five-stage wash/pretreatment processes will, in most cases, have a detrimental effect on plated deposit.
During painting or powder coating of parts, applicators sometimes have to face the challenge of elevated temperature, outgassing of the coated material during drying and curing of paint, and melting and solidifying of powder coatings.
Outgassing always exists to various degrees when material is heated. When applying such a coating directly over the base material, the major factors determining level of outgassing are porosity or micro-porosity of the material itself (more problematic with castings but also possible in HR and CR materials). Forming and stretching of the material may open some closed subsurface pores as well. Cleaning, etching and/or other pretreatment process may affect top layer of material "skin," sometimes exposing more pores.
The method of heating as well heat ramping speed and maximum temperature of the base material may influence the effect that outgassing has on final paint or powder coating appearance and performance factors.
In case of parts that were additionally electroplated with zinc or zinc nickel alloy deposit, new factors influencing outgassing are introduced. Beside naturally occurring porosity of base material, new porosity created by cleaning and etching of the surface skin of the part, electrodeposited metallic layer may both bridge and/or close existing pores in the material and also introduce its own micro- porosity. The thicker electrodeposited metallic layer is specified and applied, the more influence it will have on safe degassing of the part underneath.
It must be emphasized that all of processes are taking place in sequence of various water-based chemical baths containing a variety of acids, bases and salts which may enter such a pores and micro pores driven by capillary forces. These may react there with both material itself and also with other chemicals that it will subsequently contact. Rinsing between each process step and also final rinse will remove such a chemicals only to a certain degree. Drying of plated and chromated parts will dry surface water and may evaporate some liquid water from pores, but will also promote crystallization of any present chemicals into simple salts or hydrated complex crys- talline compounds. ,
Some examples of possible simple salts are:
- NaCl, Na2Co3, Na2Cr2O4
Also more complex or hydrated salts may be envisioned as such:
- Na2Cr0 4. 10 H20; Na2co3. H20; Na2co3. 7H20; Na2co3. 10H20;
- Na2C03. NaHC03. 2H20 or
- NaHC03 (Baking Soda)
MECHANISM AND STAGES OF OUTGASSING
The following possible mechanisms of outgassing may be identified during heating up of the surface of the parts and subsequent rise of the temperature ·of the body mass of the parts during preheating or post paint/powder coating baking.
- Thermal expansion of gasses trapped in pores of the base material and coatings
- Evaporation/gasification with multiple volume expansion of liquid water and/or other solvents trapped in pores by capillary forces.
- At higher temperatures thermal breakage of weak chemical bonds in hydrated salts and dehydration and evaporation of water with multiple volume expansion:
NaHC0 3. 2H20 +Temp - NaHC03 + 2H20 i Na2C03. 7H20 +Temp -Na2C03 + 7H20 i Na2C03. lOH20 +Temp - Na2C03 + lOH20 i
With high enough temperature some salts may start to decompose and release gaseous part as in the case of baking soda
NaHC03 +Temp - NaOH + C02 i
This replicates reactions happening during baking cookies or cake.
All of the above mechanisms and the effect on paint or powder coating will depend on the actual temperature of part, or at least subsurface of the part, and also on the stage that the organic coating is in its curing or melting and solidifying sequence. If majority of outgassing is completed before paint cures or powder melts, gases can freely leave coating and do not-affect its appearance or integrity.
If coating already started curing or powder is melted, evolving/expanding gases will have to travel through much more viscous, dense layer, leaving open channels/porosity or even crater-like eruption sites.
MITIGATING OF OUTGASING EFFECT ON COATINGS
Following precautions and procedures may help mitigate the effects of outgassing on coatings over either base material of the part or parts that were additionally plated before coating.
- Use of good quality metals with low porosity and low level of nonmetallic contaminants,
- Minimization of harsh cleaning and etching removing surface "skin" of the material which open existing pores below the surface
- In case of porous castings:
- vacuum impregnation of parts may be needed
- limited contact with low viscosity corrosive liquid baths
- pre-coating baking of parts, both plated and unplated may be necessary
- Slow ramp up of the temperature allowing gradual and almost complete outgassing before the paint begins curing or powder coating starts to melt and solidify,
- Pre-baking of the parts that exhibit severe outgassing affecting top coat appearance or integrity in either case if plated or un plated,
- Prebaking should be performed long enough to ensure maximum outgassing before final coating and baking,
- In order to achieve satisfactory results pre baking temperature may have to reach final paint cure or powder melt temperature, or be at least close enough that no additional outgassing mechanism is initiated before coating prevent free escape of the evolving gases.
Andrew K. Gruda CEF is the technical manager for Gatto Industrial Platers Inc. in Chicago. He received his Bachelor of Science and Master's Degree in Chemical Inorganic Technology in 1979 from Gdansk University of Technology; Poland. His Engineering specialization was in Corrosion and Corrosion Protection Technology. Gruda may be reached at (773) 287-0100 or via e-mail at email@example.com.