First things First
Quick solutions to problems on an electroplating line can save thousands of dollars. Experience is by far the most valuable tool a troubleshooter can possess. So successful troubleshooting begins when the line is at its best! Build your experience by walking the line when everything is running smoothly. Take note of solution color, smells, gage settings, etc. Intimate knowledge of your plating line and its idiosyncrasies will expedite the solution to future problems.

Now that a problem has developed, you must walk the line looking at temperature gages, current, anode baskets, pumps, etc. You must rule out the cleaning section of the line and the post plate section of the line. These steps may take some time but they must be done. Ninety five percent of plating problems have nothing to do with the plating bath. Occasionally a problem develops, though, which persists despite experience.

Into the Lab
Once the problem has been determined to be a result of the plating solution or the material being coated, the troubleshooting should be done in the laboratory.

Before starting a laboratory investigation, the first thing to do is ship samples of the plating bath and reject work to your supplier. Suppliers often have sophisticated labs with experienced people. Follow up with a phone call to your supplier. Speak directly with a technical service representative and discuss your problem and investigation.

Once in the laboratory, define the condition of the bath with a routine analysis and a routine hull cell. Correct any chemistry problems found by the routine analysis. The routine hull cell should be one you re used to looking at. Suggested conditions for a routine panel are:

1. A two amp, five minute, unagitated panel for acid zinc.
2. A one amp, ten minute, agitated panel for alkaline zinc.
Compare the routine hull cell panel to ones from when the problem was not present. Measure thicknesses across the panel and again compare them to past hull cell panels. If the routine hull cell panel appears normal, chances are your problem lies outside of the plating bath. Re-walk the line and review your observations. Pay particular attention to the electrical portion of the plating bath, as poor electrical connections will make the plating bath appear to be at fault. Investigate the material of the parts exhibiting the problem. Again, material problems will not manifest themselves in the lab. If you are still convinced that the plating bath is the source of the problem, continue with the lab investigation.

The next step is to use a hull cell to generate the problem. Make sure the conditions and time all hull cells were run are clearly marked on the resulting panel. Vary the conditions in the hull cell to give yourself the best opportunity to produce the problem. Some variations, which may prove useful, are:
1. Panels run at low amperage
2. Panels run at high amperage
3. Bent panels to create a shelf area
4. Bent panels to create an extreme low current density area
5. Panels run at a high temperature
6. Panels run for thirty minutes then scribed with an exacto knife (to reproduce blistering Knowing how your bath appears when operating normally will make the interpretation of these hull cells easier. Once the problem has been produced, we can proceed to the next step.

Target Identified
With the ability to produce the problem, one now needs to know how to remove the problem. The problem probably will fall into one of several broad categories:
1. Organic contamination
2. Metallic contamination
3. Poor filtration
4. Imbalance of proprietary chemicals
5. Unknown

With an unlimited supply of solution, take the opportunity to begin multiple treatments. After each of the following treatments, rerun the hull cell test, which produced the problem. First, for organic contamination, treat three hundred milliliters of solution with two grams of activated carbon. Mix the solution continuously for at least thirty minutes, then filter and run the hull cell. Second, for metallic contamination, treat three hundred milliliters of solution with one-half gram of zinc dust. Again, mix the solution continuously for at least thirty minutes, then filter and run the hull cell. Third, filter the solution thoroughly. The solution must be clear after this step. Use a filter aid if necessary. Fourth, if the solution is an acid bath, metallic or organic contamination may be affected by adding one tenth of a gram of potassium permanganate to three hundred milliliters and mix the solution for five minutes. Filter thoroughly and run the hull cell. For an alkaline solution, freeze out carbonates by putting three hundred milliliters in a lab refrigerator. Cool the solution to 30….F for fifteen minutes. Decant the solution, raise the temperature, and run the hull cell test. If one of these treatments affects the problem, you may be well on your way to solving the problem. Give yourself a pat on the back! Not too fast though. You now must translate the lab results to the production line. You must also locate and eliminate the source of the contamination. The quick results in the lab may take a couple of days to accomplish in production. But don t give up. Plug away until the job is finished.

Unknown
If the above treatments did nothing to affect the problem, things just got a lot tougher. Get on the phone to your supplier and ask for their assistance on-sight. Review their analysis of your bath. Is your bath low on carrier, high in brightener, out of balance, etc? Many suppliers have doctor solutions. By comparing notes with your supplier, they will be able to send in a new arsenal of weapons along with technical assistance. Meanwhile, there are still a few hull cells to run:

For an alkaline bath try:
1. Adding 1% sodium
hypochlorite to the hull cell
2. Adding one-half ounce per gallon of EDTA or Rochelle salts
3. Diluting the bath by 25% with virgin solution
For an acid bath try:
1. Heating the bath above the cloud point, then carbon treat
2. Reduce the pH of the solution with 50% hydrochloric acid to kick-out most organics.
Usually a pH of 2.5 is sufficient. Filter the bath, raise the pH, and add carrier.
3. Diluting the bath by 25% with virgin solution
Out of all the tests you have now run, hopefully something you can build on has emerged. If not, you are into the rare problem, which falls into the unknown classification. This type of problem will take time and effort to resolve. Calling in electricians, sending samples to outside laboratories, etc. are examples of the steps that may be necessary to solve the problem. In this case, the economics of dumping the bath and making a new one must also be considered.

Conclusion
Troubleshooting a zinc-electroplating bath will be much easier if one takes the time to observe the line when things are running well. When a problem develops, split the line into a pre-cleaning section, the plating bath, and a post-plate section. Run tests to isolate the problem to one of the three sections. When the plating bath is identified, follow these steps:
1. Send samples and reject parts to your supplier.
2. Use the hull cell tests outlined above to treat the problem. Remember, even if you can treat the problem, you will also have to eliminate the source of the problem.
3. Demand prompt service from your supplier.
4. Label or identify all tests run during the troubleshooting process.
Once the problem is solved, go back and review the troubleshooting process and learn from it. This will build your troubleshooting skills and shorten the duration of future problems

With the study of zinc electroplating process by Electrochemical Noise (EN), it was found that the EN generated during the electroplating of large conglomerate zinc deposit has large potential oscillation amplitude and positive potential drift. However, small noise amplitude and little potential drift was seen in the case of compact zinc deposit. Any metal can be plated through zinc electroplating process, but the most common are steel and iron, on which the process offers sacrificial protection.

Steps of Commercial Zinc Electroplating Process
On the commercial scale, zinc electroplating is done by the following steps.
1. Surface of the metal is cleaned in alkaline detergent type solutions, and it is treated with acid, in order to remove any rust or surface scales. Cleanliness is essential for successful zinc electroplating, as the molecular layers of oil or rust can prevent adhesion of the coating.
2. Next, the zinc is deposited on the metal by immersing it in a chemical bath containing dissolved zinc. A DC current is applied, which results in zinc being deposited on the cathode. Alkaline zinc baths are used by the finished products, to produce a more consistent zinc thickness, especially in recesses.
3. Hence an increased protection from corrosion is provided, as the corrosion of the deposited zinc is reduced. The zinc coating can increase the time required for the formation of white rust, by ten times. Finished Products also apply sealers, which are now commonly being specified by the automotive industry, further increasing corrosion protection.

It is very difficult to obtain a uniform thickness of coating, with electroplating technique. The thickness of the coating is very much dependent on the geometry of the object being plated, and it is preferentially on the external corners and protrusions of the metal body, hence not much of it is deposited on internal corners and recesses. Zinc electroplating process is used to make a clean, smooth and corrosion resistant surface. It also makes an excellent undercoat for powder coating or paint and can leave recesses on complex shaped components without sufficient zinc coating, in order to provide corrosion protection.

At the beginning of the line, the end of one coil is welded to the start of the next coil. Then there are two basic methods for continuously galvanizing sheet which differ in the way that the strip is cleaned before galvanizing-chemically or by thermal treatments. Coils of annealed cold reduced sheet may be fed directly to the galvanizing line, or alternatively, coiled sheet is continuously heat treated in the pretreatment line. After leaving the galvanizing bath, in which strip only stays for a few seconds, the surface is wiped to remove excess zinc and may be further treated to after the surface appearance, composition, smoothness or mechanical properties.

The steel sheet electroplating process utilizes the same basic principle as that for conventional decorative finish electroplating. However, the steel sheet process differs in that the electroplated coating is applied by passing the strip at high speeds through a series of plating cells, building the coating thickness by a small amount each time the strip passes through an individual cell. This continuous process for electroplating steel strip requires necessary equipment to transport the strip at high speeds (150 to 200 meters per minute and higher) through a series of individual plating cells, and is not as simple as it sounds.

An Electroplating Cell

The simplest electroplating cell is shown in the sketch where the plating solution bath is zinc sulfate.

The common schema of the electroplating cell.
This simple plating cell illustrates the actions during the plating process. At cathode (steel, for example), zinc ions dissolved in the zinc sulfate solution combine with two electrons and form elemental zinc, which deposits onto the cathode surface. At anode, water is converted to oxygen and hydrogen ions to maintain electrical balance. The oxygen forms a gas and nothing is deposited on the anode surface. The plating solution carries the current between the cathode and anode.

Plating of Steel Sheet in a Continuous Process

There are many types of anode arrangements. Some are horizontal, others are vertical, and one process utilizes a radial cell wherein the strip passes around large diameter rolls inside each plating cell, and the anodes have a radial design to match the diameter of the large rolls submerged into the plating solution. Each type of anode arrangement and design has advantages and disadvantages; thus, it is easy to see why different manufacturers use different methods. Each requires very close control of the anode-to-strip spacing to achieve efficient plating, avoiding arc spots and other defects in the coating.
Modern Continuous Electroplating Line.
Maintenance of the large volume of plating solution that is contained in all the cells is a science unto itself. Whether the plating solution for electrogalvanizing is based on zinc sulfate or zinc chloride chemistry, maintenance of the proper ranges of zinc ion concentration and solution pH are important control features. Besides plating zinc, some manufacturers have the ability to deposit alloy coatings. This requires, at a minimum, at least one more level of control of the plating solution. For example, producing a zinc/nickel alloy coating requires close control of the concentrations of both the dissolved zinc and nickel in the solution. Solution control has to be accomplished on a dynamic basis since these lines operate continuously.

Power Requirements

The electroplating process requires a large amount of electric power to deposit a metallic coating. The total power requirement is a direct function of the coating thickness that is needed to meet the customer’s specification. For example, the power required to deposit a zinc coating mass of 80 g/m2 is approximately twice that required to deposit a coating of 40 g/m2. A typical line that has the capability to process 70 to 120 tons/hour with a coating mass of 50 g/m2 will consume hundreds of thousands of amperes during this one hour of processing time. It is easy to see why power costs are major cost component for a facility that processes large quantities of electroplated sheet product.

Product Types

The most common electroplated coating for steel sheet products is zinc. Electrogalvanized zinc coatings are used by a number of automotive companies for exposed car-body panels, where the typical coating mass ranges from about 50 to 80 g/m2 per side. These coatings are considerably thicker than the electrogalvanized coatings typically used for non-automotive applications, so the lines built to make products for automotive applications usually have a large number of plating cells. Also, each automotive customer has their own specific coated-product specification.

Another attribute associated with the use of electrogalvanized coatings for automotive applications is excellent surface finish that is attainable with the electroplating process. Twenty-five years ago, when automotive companies began using large amounts of galvanized sheet for exposed body panels to improve corrosion protection, one of the few coated sheet products that could meet the demanding surface quality requirements was electrogalvanized. Hot-dip galvanized was, and still is, used for unexposed body parts. As the surface of hot-dip products improves, they continue to replace electrogalvanized sheet for exposed automotive body panels.

Other zinc electroplating lines have been built through the years to make thinner coatings. Typically, the sheet that is made on these lines has a coating mass of less than 25 g/m2. The applications for this product are often indoors; applications where the environment is not very corrosive. Many applications involve painted products. These coating lines often have the ability to apply paint pre-treatment so that the customer can paint directly without additional in-house treating.

A second type of electroplated coated-steel sheet being manufactured today has a coating composed of a zinc/nickel alloy. Typically, the nickel content is 10 to 16 percent with the balance being zinc. The unique feature of this process is that the zinc and nickel ions are co-deposited to make a true alloy coating. It is not composed of alternating layers.

The application for this product has been limited primarily to a few automotive companies. These companies have developed in-house product design and manufacturing processes to take advantage of the unique characteristics of the zinc/nickel coating. For these automotive applications, the metallic coating is often coated with a special corrosion-resistant thin organic coating on top of the zinc/nickel. The zinc/nickel alloy coating is covered by ASTM Specification A 918.

A third type of electroplated coating is zinc/iron alloy coating. The attributes of this specialized coating are somewhat like those of hot-dip galvannealed product. Like zinc/nickel alloy, zinc/iron coating is co-deposited as an alloy coating. Iron is uniformly deposited throughout the coating thickness. Also, like zinc/nickel coating, zinc/iron coating is used predominantly by the automotive industry. The attributes of electroplated zinc/iron is that it is relatively easy to weld and paint if the proper electro-priming equipment is available to the automotive manufacturer. Also, the coating is very hard, making it is less susceptible to scratching during stamping and handling. This is the important feature since the zinc/iron alloy coated-sheet product is being used almost exclusively for exposed car-body panels.

Corrosion Resistance of Electroplated Coatings

Concerning the corrosion behavior of electrogalvanized versus hot-dip galvanized coating, it is important to note that it is essentially equivalent for identical coating masses. A coating mass of 100 g/m2 will provide essentially the same amount of corrosion protection whether it is a hot-dip galvanized or electrogalvanized coating.

The reason that the automotive companies can successfully use a coating mass in the 50 to 80 g/m2 range is because they apply additional treatments on top of the metallic coating, including a zinc phosphate coating, an electro-deposited organic-based coating, a primer, and multiple-layer finishing paint coatings. Clearly, the corrosion resistance needed to protect a car body panel for over 10 years is more than that afforded by the metallic coating alone. Application of the above coatings over the electroplated metallic layer results in a synergistic system, whose corrosion resistance is more than the sum of its individual components.