Parts Cleaning Clinic: Zinc Shot Blasting, Zinc Plating Alternatives

 

Zinc Shot Blasting

Q. We are contemplating using a zinc shot blast process at a supplier to clean load bars and hooks for our powder paint line. Our process: Stage 1 Alkaline Clean (NaOH/KOH), Stage 2 Rinse, Stage 3 Iron Phosphate, Stage 4 Rinse, Stage 5 Adhesion Promoter. I hear that zinc shot blasting leaves a very thin zinc film on surfaces. I’m afraid it may be removed during travel through washer (especially stages 1 and 3, and the overflowing rinse tanks), enough to affect our zinc discharge limits for rinse water (2.6 ppm). Zinc in wastewater current runs about 1.3 ppm on average. Any suggestions? 

A. I want to commend you for thinking ahead, considering what the zinc shot blasting of your parts may do to your downstream waste treatment operations. Zinc shot blasting is typically performed on castings to improve surface finish from a sand mold casting, or to assist in the removal of residual sprue and gate pieces. The zinc source may be a pure zinc wire that is cut down to the desired size, or a cast zinc alloy that is intended to have a higher hardness with longer durability and life.

One of the suppliers of pure zinc wire shot indicates the that this method will deposit a zinc coating of about 1.3 microns on the surface of the part that is shot blast. In the case of iron castings, they indicate this is a benefit in that it minimizes the formation of surface rust on the parts that are blasted. However, in your case, this coating would likely be removed in part or completely by the high-alkaline cleaning stage or the acidic iron phosphate stage. 

Although it is likely that this zinc would be stripped off, the effect on your waste water treatment process is not as clear. These parts represent a relatively small surface area, so it is possible that you could estimate the effect though some basic calculations. By estimating the approximate surface area of the load bars and hooks and then multiplying it by the zinc thickness from above and by the density of zinc, it would give you an approximate maximum weight of zinc that would be going into your waste water treatment system (assuming it all dissolves).

Or going the opposite way, you could estimate the maximum surface area that you could run through your pretreatment line, assuming 1.3 microns of zinc on the part and all of the zinc dissolves and enters the waste stream, and that you cannot add more than 1.3 ppm additional zinc to your waste treatment system (2.6-ppm max – 1.3-ppm current).

As an example, if you currently have about 10,000 gallons per day of waste treatment flow, that would equate to about an additional 49 grams of zinc per day (10,000 gallons = 37,850 liters × 1.3 mg/L / 1,000 mg/g = 49.2 grams after units cancel). Dividing this by the density of zinc (7.14 g/cm³) and then dividing by the zinc thickness of 1.3 microns (0.00013 cm) results in about 53,000 cm² or about 8,000 in² of surface area. You would need to make the calculations for your particular situation and would likely want to incorporate a safety factor as well, but this method should give you an approximation if only the load bars and hooks will exceed this level.

 

Heating Alkaline Cleaning Solutions

Q. I have been looking into greener ways to clean mild steel in a four-stage wash system prior to powder coat application. Do you know if it is possible to completely eliminate heating of caustic solution?

A. Yes, it is possible to reduce or eliminate the heating of some alkaline cleaners. The two primary factors affecting this are the soil and lubricant (type and amount) you need to remove, as well as the cleaner formulation, which will be chosen based on the first factor. A cold-rolled steel with a light coating of a mill oil—along with minimal additional process oil application—would be the best case scenario regarding lubricant type and quantity; the lighter the oil and the less of it, the better.

The alkaline cleaner generally operates more quickly at a higher temperature, particularly if you need the caustic in the solution to saponify a fatty acid component of a lubricant. If the lubricant does not contain this type of boundary additive or chlorinated and sulfonated extreme pressure additives, the use of caustic is probably unnecessary. In this case, a more lightly built alkaline cleaner with a choice of lower temperature surfactants would likely work on the lubricants. 

An advantage with the use of nonionic surfactants is that they can be chosen with a specific cloud point. The surfactant efficiency at solubilizing oils is maximized around its cloud point. There is a detriment to significantly exceeding this temperature, since the reason the surfactant exhibits the cloud point is due to it coming out of solution; it has an inverse solubility with temperature. Therefore, if a formulation is developed using surfactants with lower temperature cloud points, they will be more effective in cleaning at or near room temperature. In some cases, additional surfactants or coupling agents are necessary to maintain the lower cloud point surfactants in aqueous solution. I would recommend talking to your current pretreatment supplier about available options if you still think your manufacturing operation is a candidate for this type of cleaner. 

 

Zinc Plating Alternatives

Q. We are an industrial fastener distributor, and last year we had a hydrogen embrittlement failure on a socket head C/S zinc-plated bolt. Our largest customer is now reviewing all options and coatings. Right now they are looking at Magni and Ecoguard dip spin processes. What are some of the best options?

A. Wherever you have a plating operation like this on steel, it can be prone to hydrogen embrittlement, unless the process includes a bake out to remove or release the hydrogen from the fasteners. The processes your customer is investigating are based on a zinc flake technology that is incorporated into an organic coating that can be clear or pigmented, typically black. These coatings can have very good corrosion resistance due to the use of the sacrificial zinc on the steel parts. In some cases, the corrosion resistance can exceed that of a zinc-plated part with a chromate or non-chromated conversion coating.

There are several job shops that apply these coatings, so you would not need to invest in this technology immediately to try it out. Perform an objective evaluation to compare the corrosion resistance and application cost of the traditional zinc plating versus the organic coating with the zinc flake technology for corrosion resistance. You may already have a sunk cost in the zinc plating technology, but should also evaluate the operational costs of each process.