Acid Wetters, Acid Inhibitors, and the Pretreatment Proces

Pretreatment is where it all begins. When electroplating ferrous substrates, it is vital to start with a strong start to ensure success throughout the process.

Pretreatment

The pretreatment process is the foundation to the success of the rest of the electroplating procedure. Improper pretreatment may lead to inadequate plate coverage, poor adhesion, premature corrosion failures, and other issues.

When pretreating ferrous metals, the typical protocol consists of an alkaline soak clean to remove surface soils. These soils can include cutting and lapping oils, stamping lubes, buffing compounds, heat treat oils, and rust preventatives. The soak clean provides for the passive loosening and removal of the soils mentioned.

Following the alkaline soak is an anodic cleaning step where the processed work is the anode in an alkaline solution. Here, the evolution of oxygen from the part surface takes on a scrubbing action and further removes the soils. The oxygen also acts to oxidize some soils, breaking them away from the surface.

Because there is an oxidative process occurring at the surface, some residual oxides are left behind in addition to any naturallyoccurring oxides that result from part storage or from the manufacturing process (i.e. heat treat scale, weld scale, rust). The most accepted process for the removal of residual oxides is the use of mineral acids. This is typically hydrochloric acid or sulfuric acid. Hydrochloric acid is usually the mineral acid of choice due to cost and availability.

Acid Wetters

Acid wetters are surfactants that are used in the acid to create a foam blanket while either stripping metal from the part or removing oxides and scales. In addition to the foam blanket that forms, the wetter also provides properties to remove residual lubricants or oils that may have adhered to the part or arrived there by dragging in on the work or the fixtures holding the part(s). These wetters or surfactants provide a charge on the soil. It moves it to the top or to the sides of the tank. It keeps it out of the way of the clean part as it exits the acid. The wetter must be compatible with processes to prevent interference with the process or the production of a surface incompatibility.

Wetters do not prevent base metal attack by themselves; extended acid cleaning or pickling can still result in surface damage. In the case of some steels, forgings, or castings, carbon used in the manufacturing of the steel is left behind. Subsequent plating processes will not adhere because carbon is a less than favorable substrate to initiate a deposit.

Acid Inhibitors

Acid inhibitors are molecular compounds that bond to the ferrous substrate. In doing this, they allow the acid to attack any oxides that may be present on the surface. They typically do not supply the detergency that a wetter may provide. However, they do prevent base metal attack during extended immersion times in the acid. Similar to wetters, care must be taken when considering the downstream processes. Many of the inhibitors can be incompatible. This can result in contamination of the subsequent plating step. It can also create surface irregularities including pitting, micro pitting, and/or star dusting.

Conclusion

There are many processes that utilize both wetters as well as inhibitors. Most of the products work well but it is critical to ensure the supplier has supplied the correct product. Likewise, it is equally critical for the finisher to completely review the process with the chemical supplier. The supplier should completely audit the operation. This is done so that the proper product is used and the anticipated results are as favorable as possible.