Acid Wetter vs. Acid Inhibitor

In the process of electroplating of most ferrous substrates the pretreatment is extremely important since this is where it all begins. The pretreatment process is the foundation to the rest of the process being successful.  Improper pretreatment may lead to poor plate coverage, poor adhesion, premature corrosion failures, and the list goes on.  In the ferrous pretreatment process the typical process consists of an alkaline soak clean to remove surface soils such as 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 work being processed 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.  Since there is an oxidative process occurring at the surface, some residual oxides are left behind as well as any naturally occurring 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 the residual oxides is the use of mineral acids, typically hydrochloric acid or sulfuric acid. Hydrochloric acid is usually the mineral acid of choice due to cost and availability. It is here where you need to make the decision between an acid inhibitor or an acid wetter and being able to distinguish the difference and applicability because they are not necessarily the same.

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 some 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, moving it to the top or to the sides of the tank therefore keeping it out of the way of the clean part as it exits the acid.  It is advisable to make sure the wetter being used is compatible with the processes down the line since they can also interfere with the following process or produce a surface incompatibility. Wetters by themselves do not prevent base metal attack, therefore 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 and subsequent plating processes will not adhere since carbon is a less than favorable substrate to initiate a deposit.

Acid inhibitors are molecular compounds that actually 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 but they do prevent base metal attack during extended immersion times in the acid.  Just as with wetters, care must be taken when considering the down stream processes.  Many of the inhibitors can be incompatible resulting in contamination of the subsequent plating step or create surface irregularities such as pitting, micro pitting, and or star dusting.

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