Electroplating involves using electric current to coat an electrode in dissolved metal cations. In metallurgy, it is one of the highest-quality procedures for achieving durable, even coats on metal substrates.
However, electroplating requires a high level of diligence, experience and precision to do correctly. It is dependent not only on the exacting specifications of the electrochemical process but also on a thorough prep job being completed before the switch is turned on. As a result, electroplating is host to several defects due to the complexity of the process.
Common electroplating defects often include issues that are present before plating, such as cold shuts, pitting, sharp edges, cleavage points and unclean manufacturing. During plating, flaking or a loss of adhesion can occur. After plating is complete, common problems include hydrogen cracking, dull and hazy deposits in the plating, blistering and oxidation. In this article, we will explain what these problems are and how they occur.
As with any complex process, electroplating should be entrusted to those who have an established reputation in the industry. Because plating is so crucial to so many industries, including telecommunications, defense, aerospace and many more, ensuring you are getting the proper plating done by the most qualified company is paramount. It can also be a frustrating process, which is why SPC is here to help guide you through it — from the best options to the full scope of pitfalls to avoid.
Let’s begin by discussing what defects can present themselves before coating.
Prior to Coating
Because electroplating relies so heavily on pretreatment for proper execution, many problems found in the final product are actually a result of problems prior to coating. Here are some of the most common sources of precoating defects:
1. Cold Shuts
Cold shuts are one of the most common defects found in materials sent for electroplating. They are the result of metal that has hardened at different stages and are visible on the surface of the metal as flow marks or lines.
More specifically, cold cuts form when a flow of melted metal reaches the cooler surface of the die. Due to the difference in temperature between the molten metal and the cold die, rapid cooling occurs in the molten metal. This causes it to harden before completely filling the mold. When the next flow of molten metal enters, it fills the remaining gaps between the die and the previous flow.
The problem here is that this new flow is now bonding with the previous chunk of hardened metal, instead of the whole piece hardening as one. This causes differential bonding that results in lines across the surface of the object. Naturally, these cracks tend to be found at the points farthest from the gate.
The most common cause of this defect is a die at too low a temperature. The effect is that the piece has inadequate mechanical strength, as fracturing will naturally tend to occur along these lines when strain is put on the piece.
Another common issue is pitting, which is essentially small holes on the surface of the object. Thicker deposits are more susceptible to pitting since they grow in size with more build-up, and re-worked parts tend to exhibit this defect more than new parts.
The source of pitting relates to the surface of the metal, though its specific causes can vary. In some cases, chemical fume suppressants can cause pitting. This is particularly true if a non-permanent type of fume suppressant has been used, though it is less often the source of trouble these days. Permanent fume suppressant of good quality will not cause pitting by itself.
Rather, pitting is typically the result of mistakes made during preparation. These can include an inadequate cleaning job or flaws on the object itself. Before plating grinding, blasting or polishing that was performed too harshly could be the culprit and can result in debris on the surface of the object. Very small holes, oxidation on the surface, microscopic fissures and pieces of non-metallic materials can also cause pitting.
Finally, pitting may be caused by stop-off residue, the breakdown of tank plumbing or lining, wire brush debris, grit from a blaster, buss arching and substances like oil left over from a bath.
3. Sharp Edges
The geometry of objects plays a large part in how they receive electroplating. The process itself relies on current, which starts a reaction on the part’s surface and attracts cations. Certain geometrical shapes will result in more attraction than others.
Due to this principle, when current passes through an object with a sharp edge, the current density increases at that point. Current distribution is of utmost importance to proper electroplating. This is why care should be taken in the early phases to reduce sharp edges, threads or bends. If not, current can accumulate in one area of the part and attract excessive amounts of coating.
When too much plating builds up on a sharp edge, the result can be a brittle layer that easily breaks. Most manufacturers correct for sharp edges by grinding and deburring the edge until it is gentler. A common rule of thumb is to round off all edges until their radius is between 0.4 and 0.8 millimeters.
4. Cleavage Points
Cleavage is the splitting of a solid along a structural plane. In electroplating, this can cause problems with structural rigidity. Cleavage points form during the die casting process.
5. Unclean Manufacturing
One of the easiest ways to guarantee problems with electroplating is by having an unclean surface on the object. This can be the result of manufacturing issues, handling, contamination or equipment contamination. A well-qualified electroplating technician will examine the object for proper cleanliness.
Electroplating attracts the coating to the object’s surface via a current. However, no matter how powerful the current, the final adhesion must occur between the surface of the object and the coating itself. If there is any impurity in between these two, adhesion will be incomplete. The result will inevitably be a coating that detaches from the object. This may result in a blister or a brittle area.
The following are common defects that occur after plating:
1. Loss of Adhesion
Is there any way to guarantee adequate adhesion?
This question is an important one, as loss of adhesion is one of the most common sources of part failure in finishes that serve both decorative and utilitarian uses. All too often, blame is cast on the electroplated coating. In reality, it is rarely the fault of the coating, but rather is the result of the surface below. Any time the surface is coated with die-release agents, oxides, alloying substances or oils, the adhesion of the plating material can be compromised.
The surface below the plating must be “active” — that is, it must be ready to receive the coating. A pretreatment system is part of the plating line and consists of presoaking in alkaline liquid, an electro-cleaning, pickling in acid, deoxidation, a descaling procedure by use of chemicals, an ultrasonic cleaning and activating strikes. These vary with the plating line and its various functions.
None of these pretreatments can last indefinitely on a plating line, and their lifespan is limited by the amount of usage employed. Because of this, one or more pretreatments may expire and fail to properly treat the object before electroplating. Examples of failings could include a depleted chemical process designed to remove a specific adhesion killer or a lead inclusion that is not removed properly from the substrate.
Several factors aid platers in preventing defects:
- Let the plater know what alloy is being used. Knowing a product’s exact alloy can be the difference between a flawless plating job and one with defects. As an example, 360 brass may contain as much as 5 percent lead, while 260 brass has 0 percent lead. Both alloys are widely used, but plating technique is different for each one. Specifically, each brass needs a different pretreatment to become suitably active. If the plater does not know what type of alloy they’re dealing with, this is less likely.
- Less powerful oils are easier to remove. In general, those oils labeled as organic — that is, ones made from living sources like animals and vegetables — can be removed without too much trouble from objects in pretreatment. However, wax and lubricants made from silicon are harder to remove and are more likely to remain despite cleaning attempts. Wherever possible, think ahead to the plating process and use oils or lubricants that are less difficult to remove.
- Make sure you heat-treat in an inert environment. It is more expensive to use an inert environment for heat treating, but the trade-off is that it requires considerably less pretreatment and, as a result, can equal out or save money. Otherwise, the pretreatment team may need to use blasting to get rid of scales caused by heat treatments. This could drive the price of a plating job by as much as an order of magnitude.
- Make sure you use plating-grade materials. High-quality materials are paramount to the finished product of a plating job. They cost more, but, as is often the case in manufacturing, the higher price can easily be compensated for by the ease of plating and lack of defects. The higher cost is due to an extra level of care in ensuring no metallic inclusions or residues are present on the surface.
2. Cracking After Plating — Hydrogen Cracking
Since the beginning of the modern industrial era, hydrogen has presented a problem for those working with finishing metals. It tends to make metals brittle, causing them to break and fracture. In electroplating, hydrogen can often work its way into parts without the manufacturer knowing.
Because hydrogen embrittlement typically presents itself after the process is over, when the part is put under stress, it is difficult to identify this problem until it occurs. Engineers cope with it by enacting several measures throughout the process. These include stress relief baking and peening the object with shot, both of which promote strength and minimize the chance of hydrogen fracturing.
3. Dull and Hazy Deposits in Plating
One frustrating defect is a dull or hazy spot on the plating. This could have several possible causes:
- Chemical Imbalance: When immersing the object in a bath, insufficient levels of Dura Additive or an excessive level of sulfate, chromic acid or contaminants can lead to dull or hazy deposits in the final product.
- Incorrect Temperature: The bath temperature needs to be between 130 and 140 degrees Fahrenheit. Use an air agitator to keep the temperature consistent throughout the bath, and try to stay within a margin of two degrees Fahrenheit.
- Incorrect Current Density: Current density should be two ASI, and deviating from this level can cause dullness or haziness.
- Incorrect Distribution of Current: If the anodes are not the proper distance from the object, the current will not flow correctly. Use conforming anodes to help with this problem.
- Improper Rectifier Function: The rectifier must be working properly to avoid dull deposits. Perform tests to look for needed repairs.
- Inadequate Preheating: Parts that are not hot enough can wind up with defects. Ensure that they have heated up to the bath temperature all the way through.
- Interrupted Current: Parts that are plated without current interruption tend to have the best results.
- Inadequate Rinsing: The rinsing process must be thorough to ensure that all cleaning agents and chemicals are absent from the object before plating.
- Part Too High in Solution: An object placed at least 4 inches below the top of the solution will receive the proper current.
- Contamination by Organic Compounds: A surface scum of organic substances can form on the bath, which can then stick to the surface of the object.
4. Blistering in Plating
Blisters result when gas expands from within the pores of the object. These gases are usually either hydrogen or nitrogen, and they find their way into the object’s surface from the lubricant applied to the die. When the object is heated, the gases expand and press against the plating, causing it to puff up in a blister. Alternatively, a lap that partially remains on the surface can cause the plating to lift off.
5. Oxidation in Plating
One of the primary reasons for plating is to prevent oxidation of the object. It stands to reason, therefore, that the object itself is capable of oxidizing before the plating is applied. Oxidation on the surface of an object can lead to poor adhesion and a lifting of the plating after the process is complete.
Contact Sharretts Plating Company for Consulting
Electroplating is a process that requires skill, experience and the right equipment. SPC is an industry leader in consulting on plating. With nearly a century of experience in the plating industry, we are prepared to help our clients obtain the best results for the best value.
Our accreditations include being the first company to be awarded the Best Manufacturing Practices Award by the U.S. Naval department, the first to be ISO 9000-certified, achieving QS accreditation and TS 16949 Tier 1 and Tier 2 Automotive Quality Management standards.
For experience you can count on, contact SPC for electroplating consulting.