Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
Plating Defects- Caused mostly by the items being plated
The condition of the surface to be coated has a major influence on the quality of the plated deposits. Some are obvious such as visible scratches, pits, rough surface, discoloration as from heating or colors used for fabrication, dissimilar metals, overlapped joints, sharp edges, metal splinters, dents and highly irregular shapes, pockets that can entrap foreign materials difficult to clean such as spot welds, arc welds that are porous or cracked, magnetic metals that hold particulate matter. Some defects are only visible with a microscope but can also cause plating defects, such as microscopic particles slivers, tiny pits, surface inclusions and tiny pores. Metal splinters are particularly onerous because many of them can protrude upward, and be completely plated over, but with their point covered by only a small amount of plated deposit. These are sources of corrosion failure in corrosive environments.
Additional problems are caused by dissimilar metals in contact, gas entrapment in castings, burns, slag and residual magnesium. Powdered metal fabricated items always contain numerous pores. Porous items may have contaminants in the pores that bleed out during the plating process. Pores can entrap cleaners, and acid treatments that are difficult to rinse out. Repeated hot and cold rinsing should be used to try to remove residual contaminants from the holes. The temperature of the hot cycle should be significantly above the highest temperature or the plating or processing solutions.
In cleaning, sometimes grease and oils are removed, leaving particles tightly held to the surfaces. This is often true when a solvent is used. Most solvents remove oil and grease very well but leave particles and splinters on the surface making it difficult to impossible for the alkaline cleaners to remove them. In many cases it is best not to use solvents. The alkaline cleaners will remove both oils and particles. Electro cleaners help remove particles. Anodic current is best for most applications, but direct (cathodic) cleaning produces 2X the amount of gas bubbles and may be needed for particle removal. The use of both Cathodic (direct) followed by anodic cleaning works well. There mild acid cleaners that work will where applicable.
Items to be plated that have deep recesses or irregular surfaces, including protrusions that will be close to anodes in the tank, require shields or robbers to even the current distribution, to prevent thick and thin areas, and/or burning.
Aluminum has the potential to have defects due to the plating process. Over etching can occur due to alkaline cleaners with high pH, alkaline etching, too long in the deoxidizer or too long in the long zincates. Each step can cause pit defects and if long times are used in each, it is likely to cause pitting defects in the plated deposit.
Inspect items to plate to determine what, if any, defects are present and determine how to deal with them.
Know that contaminants entrapped in crevasses, or holes in the surface could bleed out in the plating process and have a negative effect on the plating solution.
Use alternate hot and cold rinsing following each process step to remove materials entrapped in the pores.
Keep anode area as constant as possible.
For nickel solutions use polypropylene bagged titanium baskets where possible. Keep anode bags full. Bar anode slivers should be removed or grouped together. Most plating solutions used bagged anodes. Keep bags clean.
Keeps anode length a little above the lowest part of the items being plated for more even current distribution?
Use robbers or shields when plating irregular shapes.
Be aware that there some reactant grinding lubricants that can only be removed by abrasion. Solvents and alkaline cleaners will not remove them.
Do not allow particles to spill over the top of the anode bags. Insoluble sulfur and metal particles can spill over the top of anode bags. Co-deposit of particles resulting in roughness of the plated surface. Also sulfur compounds can co-deposit from anode materials resulting in metal deposits that are brittle causing failure in the deposit. Heating of the pated item can result in cracking of the deposit can cause cracking failure in the substrate.
When plating items that show defects, deal with the consequences of plating surfaces and how they may affect the plating solution and the finish of the surfaces. If possible deal with the defect, by filing holes smoothing our rough areas, etc. don’t plate items with defects without understanding the consequences and have communicated with the user of the items.
Solvent clean prior to alkaline cleaning if at all possible and when it is appropriate, such as when a heavy coating of grease or oils. Be aware of solvent cleaning because it may leave insoluble particles that do not come off in the alkaline cleaner even when direct or reverse current cleaning follows prior to plating Also solvent cleaning may leave insoluble particles that will not be removed by direct or reverse current cleaning is used. Wiping and/or brushing may be required in some cases.You may download this article FREE in .pdf form, save it or share it with a colleague. Click here.