With an estimated 2,300 plating job shops in the U.S. and an additional 5,000-7,000 captive shops and overhaul facilities performing plating and finishing operations, it is not surprising that there is a need for a variety of specialized stripping processes capable of removing metallic coatings.
Highly specialized coatings such as low-temperature alloys on electroformed parts, flame spray coatings, anodized coatings, copper/nickel/chromium on plastic and engineering brazing alloys require very specialized removal technologies. How to remove these coatings is beyond the scope of this article.
Metallic coatings are removed for remedial reasons or as part of the manufacturing or rework process. There are four basic reasons to strip metallic coatings:
1) To salvage parts having plating defects such as blisters and skips.
2) As a manufacturing step such as solder stripping in printed wiring board production or removal of heat treat copper stop-off.
3) For overhaul or rework such as removing brazing compounds that secure blades in gas turbine engines for compressor rework.
4) Stripping plating racks and fixtures.
While mechanical stripping has application in some industries, it is not commonly used in the plating industry for several reasons:
In chemical stripping, the coating to be stripped must be oxidized from the metallic state to an ionic state. Therefore, an active ingredient in any stripper must be an oxidizing agent. Depending upon the type of stripping process (immersion or electrolytic), the oxidizing action comes from the use of selective chemistry, electric current or both.
Selectivity is required to prevent attack on the base materials while allowing complete stripping of the metallic coating. Selective stripping can be achieved in several ways:
1) If the metal to be removed is more electrochemically active than the substrate (for example, zinc on steel), then selectivity can be obtained by using various electrolytes such as inhibited hydrochloric acid or sodium hydroxide (caustic soda).
2) By incorporating chelating or complexing agents in the stripping solution that have a greater affinity for the metal being removed than for the base material.
3) By adding inhibitors to stripping processes to chemically or physically adsorb onto the substrate and “protect” it from the stripping action. For example, to remove a nickel deposit from copper-plated steel while leaving the copper intact, organic sulfur compounds are added to the stripping solution.
4) By using special activators to help initiate the stripping action on the plated part without damaging the substrate. This is especially important if the plating is aged or heat treated such as electroless nickel deposits on steel.
Other components found in metal stripping solutions include rate accelerators, electrolytes (for conductivity) and water.
Effect of Water
It is not uncommon for the water content of a stripping solution to have a dramatic impact on the solubility of the solution or the inhibiting qualities of the process on the substrate. Platers often use concentrated nitric acid to remove nickel deposits from steel substrates. Although this can be fast and highly effective, it can be counterproductive. If the concentration of the nitric acid in solution decreases through use or the accidental introduction of water, changes in the degree of ionization of nitric acid occur. This makes the acid much more aggressive toward steel substrates.
As alkaline stripping solutions age, byproducts build up and metal content increases. This upsets the ratio of water to other components. If the water content of the stripping solution is not adjusted, stripping action is retarded or ceases. In these situations a portion of the stripping solution should be removed and the solution replenished with new chemistry. This usually extends the solution’s life.
Immersion vs. Electrolytic
The methods for selectively removing metallic coatings can be broken down into two general classifications: chemical immersion or electrolytic processes. Within these classifications, the solution chemistry can be acidic, alkaline or neutral, and the applications can be described as either for parts or for racks and fixtures. Electrolytic stripping processes were among the first used by the finishing industry and are regaining popularity due to their more environmentally friendly nature. However, chemical immersion strippers are the most widely used today.
Acidic Immersion Stripping
For acidic immersion stripping, concentrated or dilute mineral acids are used. Unfortunately, mineral acids are generally not as selective in coating removal as are their alkaline counterparts. In situations where mineral acids can be employed for removing metallic deposits, it is generally recognized that the controlled addition of small amounts of fluorides, chlorides, bromides or organic acids will help accelerate and prolong the stripping action. There are a number of stripping processes designed around mineral acids that contain these ingredients plus inhibitors to enhance selectivity.
Advantages of acidic strippers include their fast rate, high metal-holding capacity, relatively low cost and ease of waste treatment. Disadvantages include their aggressiveness on racks, equipment and ventilation systems and the limited number of base metals that can be stripped. These drawbacks have helped promote the use of less aggressive alkaline processes.
Alkaline Cyanide Immersion Stripping
The earliest alkaline chemical immersion stripping processes were cyanide based. Even with today’s stringent regulations for handling, waste disposal and discharge, there are still a significant number of facilities that use cyanide-based technology for both plating and stripping operations. The major advantage of cyanide technology is that the chemistry dissolves most plated deposits while leaving steel substrates intact. Another plus is that parts can be routinely left in the process solution overnight and, due to the low-temperature operation (room temperature to 140°F), damage to substrates is minimal. Cyanide-based stripping solutions use sodium hydroxide (caustic soda) as a source of alkalinity and protection for the steel substrates.
Cyanide is the complexing agent that helps to remove plated deposits into solution. Nitro-aromatic compounds are the source of the oxidizing power.
The use of cyanide-containing stripping agents began declining in the 1970s as industry moved towards safer process chemicals.
Alkaline Non-Cyanide Immersion Stripping
This constitutes the largest commercially used process today for removing metallic coatings. The two largest applications are stripping of electrolytic and electroless nickel and the removal of copper deposits.
Copper deposits have traditionally been removed using sulfur-rich caustic solutions (followed by a secondary cyanide dip to remove the copper sulfide film that was formed) or with cyanide-based processes alone. This process quickly fell into disfavor when single-step, non-cyanide processes were developed.
Whether in the electronics industry or the plating industry, faster methods were sought to remove copper deposits. Several commercially available, popular processes include ammoniacal persulfate and ammoniacal chlorite solutions. In either case, the ammonium ion provides sufficient complexing power to draw the copper into solution. The chlorite or persulfate provides the necessary oxidizing power to change the copper metal to an ionic species. Both of these processes provide a fast, safe method removing deposited copper. However, they require more control than earlier cyanide-based technology and pose their own waste treatment and ventilation problems.
Alkaline non-cyanide metal stripping technology also provides a means for the removal of both electrolytic and electroless nickel deposits from steel, brass, copper and zinc die castings. Typically, these process solutions consist of an amine compound to complex or chelate the nickel metal being brought into solution, an oxidizing agent and ingredients to accelerate the stripping action while promoting passivation of the base material. Unlike their cyanide counterparts, these alkaline non-cyanide processes generally run at elevated temperatures (140–195°F) and have a much faster stripping rate.
Additionally, some processes are very effective in removing high-phosphorous electroless nickel deposits.
In general, immersion chemical stripping processes are the most widely used. They have fewer variables to be controlled and are less expensive to set up than their electrolytic counterparts. Immersion strippers are also less likely to etch or pit sensitive substrates and can be used on parts of all sizes and shapes whether in rack or barrel applications. Their major drawbacks include limited solution life, makeup and disposal costs and the need to segregate the rinse streams to prevent chelation of metals present in the waste stream.
The earliest stripping processes were electrolytic. These processes consist of either cyanide-based plating solutions where the workpiece is the anode and the coating is redissolved in the plating solution or where a strong mineral acid, such as hydrochloric, is used to strip coatings anodically.
The equipment requirements and set up are much more expensive than for chemical immersion strippers. Electrolytic stripping is not able to easily handle complex geometries (due to high- and low-current-density areas) or parts that have to be barrel processed. Electrolytic stripping also tends to etch or pit sensitive steels such as those that have been heat treated or have high carbon or alloy contents. Overall control of the process is much more difficult to maintain.
However, anodic electrolytic strippers are not without their redeeming features. These include extremely fast stripping rates, minimal buildup of reaction or by-products, relatively inexpensive operation and the ability to strip multiple coatings during a single operation. The largest commercial use of electrolytic strippers is for processing rack tips that have heavy metal buildups.
In fact, today there is a growing trend to using electrolytic rack strippers in place of the less costly mineral acids because mineral acids tend to degrade the plastisol rack coatings much more rapidly. Furthermore, electrolytic strippers can be desludged, extending their solution life over mineral acids, thereby reducing disposal costs. Another benefit of electrolytic stripping processes is that they generally contain very little, if any, chelating agents. This makes electrolytic strippers more readily acceptable for in-house waste treatment processes.
In recent years, more and more captive plating facilities have been investigating the use of electrolytic stripping processes to minimize disposal cost. With proper fixturing, today’s commercially available electrolytic stripping processes can remove bright electrolytic nickel deposits. This has helped some manufacturers realize significant cost savings where previously rejected components would be scrapped due to the expense of removing coatings through immersion processes only.
Preparation for Stripping
Stripping metallic coatings for overhaul or rework of parts and stripping racks and fixtures are planned manufacturing steps. As such, they are controlled processes. Stripping of parts for recovery because of plating defects is usually unplanned. As such, control and preparation are usually minimal.
When parts require stripping to remove a defective plate, there is usually a rush to get the job done. Improper cleaning or activation of a plated component destined for the stripping tank can often be the result. This will cause excessively long process times, incomplete stripping and/or pitting and etching. It is imperative that any organic film (fingerprints, rust preventive oils, etc.) and shop soils be removed from the component in a suitable alkaline cleaner. Especially in the case of nickel or electroless nickel deposits, the coating must also be properly activated.
A typical pretreatment cycle prior to metal stripping usually includes a 3–5 min soak in a hot alkaline cleaner followed by thorough rinsing and cathodic activation in an electrocleaner. This is followed by acid activation in hydrochloric or suitable mixed acid. This is especially true for high-phosphorus electroless nickel deposits or deposits that have been aged or heat treated. Typically, a properly activated component will begin to darken almost immediately after immersion in the stripping solution. Non-activated parts take up to two hours to dissolve the oxide films before initiating the stripping action.
Once a part has been properly cleaned, activated and stripped, it is not uncommon to have smut remaining as a result of the stripping action. This is especially true of the alkaline, non-cyanide strippers for nickel and copper. With the move from cyanide and chromic acid (traditionally used for desmutting), more and more reliance is on a two-step process. Typically, after stripping, parts are rinsed thoroughly and immersed into an inhibited acid and/or mineral acid salt combination. This is followed by a thorough rinse and cleaning in a good alkaline electrocleaner. Usually, this two-step approach will loosen up any smut formed on the surface of the component and quickly remove it with the electrocleaning action.
Another consideration when processing components through a stripping solution is when components are stripped, the base material is extremely active and prone (especially on humid days) to flash rusting. Therefore, it is important to remove the components as soon as stripping is complete. A quick and thorough rinse is then followed by either a rust preventive dip or an alkaline film to preserve the integrity of the base material. If stripping is done in-line (or in an adjacent off-line operation), the part may be placed right back into the plating cycle.
Equipment and Control
Chemical stripping processes have certain equipment and control requirements. Far too often the stripping process is tucked away in some dark corner of the plating shop and consists of hand-me-down equipment.
Chemical immersion strippers require the least amount of equipment. Generally, a process tank fabricated out of a suitable material, a reliable temperature controller and heat source, ventilation and solution agitation is all that is required.
Most stripping processes require elevated temperatures. This can lead to thermal breakdown of sensitive components if solutions are allowed to overheat in the area of the heating source. Additionally, insufficient solution movement, especially in baskets, will cause a chemically depleted zone around the components, which can lead to changes in pH or solution chemistry. This can result in etching or pitting of base materials and/or no stripping action.
Generally, solution movement via a mixer or an in-tank pump will more than suffice and can significantly shorten process times.
All electrical equipment (temperature controllers, heaters, mixers) in a stripping solution must be properly grounded. Stray currents can set up galvanic cells in a strip tank and cause etching of base materials. All tanks, steam lines and related equipment connected to the tank should be electrically insulated from stray currents originating from other locations in the shop and effecting the strip tank. Also, mixers should have a non-conductive coating applied to the shaft and prop to prevent electrical leakage from the mixer motor entering the solution.
Unless recommended by the manufacturer, air should not be used to agitate a stripping solution. First, this will increase the amount of airborne chemical which may prove to be irritating to personnel or damaging to your equipment. Second, air may oxidize active materials in the process solution and prematurely shorten its life cycle.
Lack of process control is a primary cause for premature demise of stripping solutions and etching or pitting problems. Chemical stripping processes, like plating processes, generally have temperature, pH and chemical limits imposed on them by the manufacturers.
Typically, a 10–20°F increase in process temperature will significantly increase the stripping rate. However, certain oxidizers, inhibitors or chemical accelerators may be temperature sensitive. Exceeding the manufacturer’s recommended temperature may cause breakdown of these components and lead to inadequate stripping or base metal attack.
Likewise, the pH of the process solution may also require periodic checking to ensure that upper or lower limits have not been exceeded due to dragin, dragout and consumption. This could result in poor stripping or base metal damage.
Although metal stripping solutions are made to strip and contain metal ions in solution, many of them are susceptible to metallic contamination. This is especially true of the alkaline, non-cyanide nickel stripping processes. These can often be contaminated with cadmium, copper, chromium or silver from brazing alloys, diffused coatings or racks that were used to mount the parts to be stripped. Contamination levels as low as 100–200 ppm of any of these metals can reduce or stop the stripping action.
Even with good equipment and proper solution control, problems can exist when removing metallic coatings. High-carbon or alloyed steels are much more susceptible to pitting or etching than are mild steels or low-strength alloys. Additionally, stresses caused by deformation of components can put up localized cells, which may show up as etching during the stripping operation. Non-uniform metallurgy can often complicate situations by causing erratic pitting or etching. Other problems include the processing of dissimilar metals that have been joined and plated. The component with the higher electroactivity may end up acting as a sacrificial anode in a stripping medium and be damaged. Also, parts that have undergone processing resulting in precipitation along grain boundaries are more susceptible to attack at the grain boundaries than are homogeneous substrates.
The technologies for selectively stripping many of today’s metallic deposits such as copper, zinc, nickel and electroless nickel, are proven and an integral part of manufacturing and repair processes. Ongoing demands on metallic deposits to provide increased functionality and/or reduced environmental liability have spurred the growth of diverse electrolytic and electroless alloy plating solutions as well as thermal spray coatings. The growth of these newer coatings has increased the need for better stripping processes, which will effectively remove these newer coatings at faster rates and with less environmental impact.