Lightweight construction and electroplating problems - Part 2 Resulting electroplating problems

The atoms in the stress field of a dislocation detach more quickly from the surface and leave behind so-called etching pits. In addition, mixed potentials always form on alloys [5].

The effect of precipitates on the electrochemical potential of small areas, which can lead to grain decay, is important for surface treatment. The potentials of intermetallic compounds cannot be determined as mixed potentials of the metals forming them. As they have different bonds, they also have different energy contents and therefore their own potentials and exchange current densities. These can only be determined by measurement.

As can be seen from the compositions, the intermetallic compounds can be quite complicated.

The fact that they have different chemical bonds to the component metals means that they also have a different electrochemical potential. As a rule, they are more noble due to the stronger bond. However, this varies from metal to metal. These special bonding ratios also mean that the expected predictable mixed potential does not develop.

Troubleshooting

When searching for defects, the geometric distribution of the occurring defect across the workpieces must always be determined first (size, number, distribution). This often rules out the possibility of looking for the fault in the electroplating process. Why? If the defects are systematically distributed, they cannot originate from the electroplating process. Electroplating always produces irregularly distributed faults. Regularly distributed defects always originate from prefabrication. The electroplating process can at most have amplified them.

For example, tiny "bubbles" appeared on every second component in a row. Electroplating cannot do that. So it was good to know that the components were always punched in pairs. This made it clear that the defects were caused by the stamping process. Close inspection also showed that they were not bubbles, but round bumps. What had happened? Ballotini (small glass balls) had been used in the stamping shop. A ball had pressed into the stamp and then fallen out again. This dent then embossed the elevation on the components, which was initially identified as a "bubble in the gold layer".

The same applies if defects always occur in the same place on a workpiece.

The "qualifier" must also be familiar with the characteristics of the respective electroplating process. Silver is normally deposited with a one hundred percent current yield. This means that no hydrogen pores can form. After a short storage period, circular, gray spots formed on the white silver layer. Observation under the microscope showed that there was always a pore in the middle. The gray spot was caused by electrolytes diffusing from the pore. The base material was porous.

Bubbles and flaking are not necessarily caused by a lack of cleaning. Such suddenly occurring adhesion problems of the gold layer on FeNiCo were not caused by electroplating. The metallurgist had changed the cobalt mine in the Congo. The ore contained slightly more chromium. Although the composition of the material was still correct according to DIN, the weak chromium passivity on the surface was sufficient to cause the defect.

In addition to determining the defect distribution, it is necessary to define the defect pattern as precisely as possible. The following list can serve as a guide [3].

After these two initial analysis steps, it is essential to first determine the process step(s) that could be the cause of the error, e.g. using the list. Of course, the first thing to look at is your own process. However, it is extremely damaging and expensive if you make a mistake and start by changing the data of the process that is running well. The above example of the small change in the composition of the raw material shows how important this is.

It is also important to provide the customer with exact error patterns. In order to save costs, you want to react quickly. It is not very helpful if the customer says "surface defects" or "the parts cannot be processed". A relatively long time then elapses before the faulty parts are available in-house and the exact fault analysis can be carried out.

List of defects

Flaking, torn layers that usually roll outwards are caused by tensile stresses above the tensile strength with low adhesion. They are usually caused by inadequate cleaning before the metal layer is deposited or by passive materials on the surface.

Build-up, strong, localized accumulation of material due to a strong increase in current density at points protruding from the surface (edges of grooves, tool marks, burrs, etc.).

Bubble, detachment of the coating or the intermediate layer with bulging of the coating material due to excessive compressive stress in the coating and/or gas accumulation (hydrogen) with simultaneously insufficient adhesive strength.

Defect, limited, non-galvanized area due to an area covered with an insulating material (passive layer, gas bubble) or a material with higher overstress.

Lack of adhesion, separation of the layer from the base material during use as agreed or a comparable load (agreed test) due to insufficiently removed layers on the base material.

Pimples, isolated roughness scattered over the surface due to ingrowth of particle impurities on the surface.

Pores, usually a circular depression with a steep wall (possibly with raised edges) caused by a non-conductive inclusion (inclusion, bubble, pore) in the base material.

Cracks, linear separation in the layer, perpendicular to the surface due to a crack on the base material surface. Can be caused by deposition in the case of brittle coating material and high tensile stress (e.g. Rh, Cr, Ni).

Hydrogen embrittlement, failure of the workpiece, especially with high-strength material as a result of overstressing of the bonds due to the penetration of hydrogen. The cause is difficult to determine, as hydrogen can penetrate throughout the metallurgical process and during corrosion. Annealing alone after electroplating is not sufficient.

Deposits and indentations, surface markings (elevations or depressions), which may be irregularly distributed or regularly recurring.

Rolling on, flakes of a softer metal that have been stretched in the rolling direction during rolling and are more or less firmly bonded to the surface.

Bending number, ductility measure, test transverse to the rolling direction.

Bubble in the base material, cavity of round or elongated shape enclosed on all sides by the material, usually with a bare inner surface. Bubbles near the surface can appear as visible bulges or be torn open.

Bubble nest, accumulation of mostly smaller bubbles.

Doubling, splitting, internal material separations that are only rarely visible on the unprocessed semi-finished product and appear on the surface, but stand out after electroplating.

Inclusion, non-metallic inclusions (oxide, slag, sulphide) in the structure, can emerge on the surface.

Rolling-in, particles of a harder material than the base material pressed into the surface during rolling.

Flow patterns, flow and buckling lines, lines created by localized flow of the material during cold forming or semi-finished product processing (e.g. deep drawing).

Flakes, scales, irregular, flat surface separations of small thickness, which appear as numerous fine shells, they can be stretched in the rolling direction and are connected to the base material. After their detachment, a scar usually remains on the base material.

Microstructural inhomogeneities, adjacent areas of different phases or different composition, the areas have different physical and chemical properties.

Annealing plumes, annealing spots, surface discoloration caused by air or the effect of other media during annealing due to the formation of other compounds (salts, oxides) in these areas.

Burr, material remaining in the cutting direction after a cutting process with an expansion exceeding the specification value.

Burrrib, burr protruding from the surface due to a gap between the die parts during primary forming or forming or due to material pressed out during resistance welding.

Pits, seizures and small holes scattered over a surface area.

Decoiler marks, torn surface or scratches of varying width, depth and concentration, usually more or less evenly repeating across the rolling direction due to relative strip movement during decoiling.

Cold strip scuffs, grooves, scratches, abrasions or tears of varying size, preferably in the rolling direction, which may be open or closed and free of metallic inclusions.

Grain size, non-compliance with the specified guideline value, insofar as it negatively influences the usability.

Corrosion, corrosion products visible on the surface, caused by the effects of unsuitable media during transportation or storage.

Blowholes, indentations in the surface of the unmachined part caused by gas bubbles or segregations that have fallen out.

Lack of polishability, formation of etching pits and other defects during chemical or electrochemical polishing.

Pore, depression with a small surface area and steep slope, with the edges lying in the plane of the reference surface.

Chatter marks, wave-like irregularities with short periodic intervals, transverse to the direction of transport.

Rough surface, excessive roughness of the surface, resulting in a surface that is too rough even after electroplating.

Crack, localized separation of the material structure of small width, but often considerable depth or length.

Slag line, non-metallic inclusions that have been stretched during rolling.

Dirt, light to heavy deposits of particulate and/or liquid foreign matter on the surface due to improper handling during processing, storage, transportation.

Segregation, accumulation (crystallization) of foreign material from the melt on the surface of the raw part.

Rolling groove, straight, trench-shaped processing track of greater length in the processing direction.

Hydrogen content, excessive content of the workpiece from the preliminary stages.

Depending on the customer and his processing of the electroplated workpieces, further adapted defect patterns must be generated.

A correlation between electroplating defects and metallurgical causes can be seen in Table 3.

The electrochemical and therefore electroplating-relevant phenomena associated with the hardening processes have already been described in the hardening methods.