Question: We silver plate aluminum in the drum. We chemically apply nickel as an intermediate layer. After silver plating, we notice a sharp edge. As we could not find any faults with the electrolytes, we cannot think of any other options. Do you know of any other possible causes for this problem?
Answer: Sharp edges are actually unusual or rare after coating, as the edges are rounded off by coating. The drum rotation - which is usually somewhat slower than usual with aluminum - should also cause rounding rather than sharpening, if it has any influence at all.
As we unfortunately have no further information on the parts, we would at least question the process from start to finish, especially as we do not know how high the proportion of defects is.
Starting material
The first step is to examine the initial condition. The edges may already be very sharp here - at least some of these edges. Depending on how the parts are manufactured, there may still be fine burrs or overlaps that make the edges appear less sharp. Subsequent processes - such as the pickling process for aluminum - may have removed these and revealed sharp edges.
Pre-treatment
During the pre-treatment of aluminum, material is usually removed to a greater or lesser extent. This applies not only to the burrs and overlaps mentioned above, but in general. You should therefore check the parts immediately after pre-treatment. If you find sharp edges, you will have to adjust the treatment times. We are aware that this involves a risk with aluminium, as you have certainly optimized the pre-treatment times for your products over a long period of time. Pickling inhibitors could offer a possible compromise.
Ni-P layer
A chemical nickel layer as an intermediate layer is a good choice in many cases. The adhesive strength is usually further improved by a chemical pre-nickel. However, practice often shows that various problems can occur during subsequent silver plating. This affects both the adhesive strength and other layer properties. To overcome this - and at the same time further improve corrosion resistance - electroless nickel-plated aluminum parts are often additionally electroplated. While the chemical nickel layer largely reproduces the surface - and therefore also the edges - an electroplated nickel layer could help to round off any sharp edges that may result from the pickling process.
Silver plating
There are several known defects in silver plating that can either lead to sharp edges or at least to problems that can be mistaken for such. These can be rough deposits in the high current density range (edges), dendrites and smaller outgrowths.
Rough deposits can usually be traced back to particles in the electrolyte. Dendrites often occur on matt surfaces, but are also known from highly polished parts. A thicker intermediate layer often helps here. If not, the cause can be found in organic degradation products. Activated carbon treatment and, if necessary, the addition of up to 0.1 g/L potassium iodate can help here - although an improvement only occurs after a few hours of production.
Some substrates - such as brass with a high proportion of beta phases - are known to have a negative effect on the build-up of silver layers. A thicker intermediate layer or an electroplated nickel layer in general could also provide a remedy in such cases.
Tempering
Tempering can have a strong influence on the properties of aluminum and subsequent coatings. It depends on when and how this is carried out and which alloy is involved. Adhesion tempering is often carried out after chemical nickel plating. These are carried out at 200 °C for 1-2 hours. This is also known as baking. The parts are then further silver-plated. In the final inspection, such tempering is also carried out as an adhesion test, but the dwell time in the tempering furnace is several hours.
Thermal post-treatments are also carried out at temperatures above around 280 °C in order to increase the hardness of Ni-P coatings from the original 500 to 600 HV to over 1000 HV. This usually starts at 400 °C. Annealing at 650 °C for at least 10 hours significantly improves the corrosion resistance of the coatings.
At temperatures below 250 °C, the coatings only tarnish slightly. Between 250 and 700 °C, this can only be avoided in an inert gas atmosphere or by working in a vacuum.
Sometimes aluminum is annealed before coating to reduce internal stresses. If the aluminum is under high internal stress, this can lead to a change in the crystalline structure of the material, which in turn can affect the coating. This influence can already take place during pre-treatment, as the stresses can affect the speed of chemical reactions.
The correct temperature and time depends very much on the alloy. In the case of aluminum-copper alloys and aluminum-magnesium-silicon alloys, prolonged annealing at 200 °C can lead to precipitation hardening. This is not to be expected with pure aluminum. For cold-worked aluminium alloys, the temperature of 200 °C may be too low to trigger complete recrystallization.
The process of stress relief annealing, a controlled heat treatment process that aims to reduce internal stresses and thereby improve ductility and workability without significantly altering strength and hardness, is well known. This helps to increase the service life and reliability of aluminum components. The workpiece is heated slowly and evenly to a temperature between 250 and 350 °C. The exact temperature depends on the specific aluminum alloy and the existing stresses. Slow heating is crucial to avoid thermal shock effects that could create new stresses. The aluminum is held at the target temperature for a specific time, typically between half an hour and several hours. This holding time allows the internal stresses to dissipate through plastic deformation and recrystallization processes. The holding time depends on the thickness of the workpiece and the extent of the stresses. After the holding time, the workpiece is slowly cooled down to room temperature. Slow cooling is important to prevent the formation of new stresses. The cooling rate can be controlled by the ambient air or controlled cooling methods. The primary effect of stress relief annealing is the reduction or elimination of internal stresses. Among other things, this also improves the dimensional stability of the workpiece.
You would have to determine the correct temperatures and times yourself in tests. As a starting value, we would suggest 300 °C and a holding time of six hours.
