Get It Plated Right
This fact sheet series is produced by the Minnesota Association of Metal Finishers & Minnesota Technical Assistance Program for metal fabricators and their platers.
Selecting Materials for Plated Parts
Fact Sheet #3
The type of metals used to construct a part can affect how easily it is cleaned and the part’s physical condition after a finishing process. If dissimilar metals are used without regard to how the combination reacts to finishing chemicals, severe problems can result.
Dissimilar metals are typically brought together through joining processes. Problems with welded, brazed or soldered assemblies are most common, but bolted and press-fit assemblies must also be dealt with carefully.
Mixed Steel and Stainless Steel Assembly Triples the Plating Cost
A manufacturer of food packaging machinery attempted to lower costs on one of their larger weldments, by welding stainless steel plates to a cold-rolled steel tube. The assembly was sent out for electroless nickel plating to protect the steel tube from corrosion during use. By oversight the plater was not told the plates were stainless steel.
When the plated parts came back, the manufacturer found small blisters forming on the stainless steel. Plating in these areas easily scraped off. But, the plating on the steel tube was acceptable. The manufacturer called their plater and learned electroless nickel–as well as most other types of plating–does not bond well to stainless steel.
To salvage the parts, the plating was stripped and the parts received a nickel strike followed by replating with electroless nickel. The total cost was three times the original quote. If the plater had been able to prepare for the dissimilar metals, a nickel strike step could have been inserted into the process, giving adequate bonding to the entire part, for about a 15% increase in plating costs.
Problem Sources and Solutions
The manufacturer had problems when their plating shop had insufficient knowledge about the part they were finishing. The problem could have been avoided through better communication when the job was arranged. Alternatively, this part could have been made entirely out of cold-rolled steel and plated, or entirely made of stainless steel and left without plating, avoiding issues of adhesion entirely. The best choice depends on overall economics and how the part will be used.
The selection of the base metal of a part and the type of plating applied have a strong influence on the plating bond strength. Given good preparation and processing the bond strength for substrates and finishes vary according to the following table.
In addition, the bond strength of plate on a hardened alloy will be lower than for the same plate on the same unhardened alloy. The problem with chrome alloys, and to a lesser extent nickel and leaded steel alloys, is that these metals oxidize in the presence of air. The same thing that gives chrome alloys corrosion resistance, makes the surface difficult to activate and creates weak bonds with the plated metal. Weak bond strengths can be partially compensated for by increasing the thickness of plating.
Welded stainless steel assemblies are troublesome for two reasons: 1) plated coatings do not bond well because of its chromium content; and 2) welding causes further oxidation and changes the local metal composition slightly.
Stainless steels and some other difficult metals can be plated successfully if surfaces are prepared with a thin, nickel or copper strike. However, plating bond strength will be roughly half as good with carbon steel. Also, strikes are off-line, specialty plating baths. They are always an extra process step, used with another plating bath which produces a more corrosion resistant, wear resistant, or aesthetic finish. Another disadvantage is the nickel strike coating does not penetrate into recesses, holes and tubing nearly as well as electroless plating. If plating quality is important on these hard to reach surfaces, the part design or metal selection needs to change. When strikes are required, plating costs increase.
Dissimilar metals in an assembly can also cause problems, especially when that information is not communicated to the plater. Frequently one of the metals will etch or corrode in the strong plating chemicals, which would not occur in the absence of the other metal. The dissimilar metals plus the plating chemical set up an electrochemical corrosion cell. This can result in: stains; a roughened texture in the corroded areas; an unintended metal coating on other metal surfaces, affecting either the part appearance or the plating quality; or in extreme cases, substantial loss of material in the assembly.
- Change welding rods when jobs change, to match the base metal of the new job. If an operator continues to use stainless steel wire or rod for carbon steel parts, in general the weld will perform well structurally, but plating on the welds will flake. Other metal mismatches will generally perform better than combinations of steel and stainless steel, but adhesion will be compromised.
- Use only stainless steel brushes and tools to remove slag and scale from welds on stainless surfaces, to prevent imbedding impurities which can create defects over time.
- Avoid brazing with copper-zinc filler on ferrous parts that will be plated. Use silver solder or join components after plating. Copper dissolves in acid baths and leaves a thin immersion copper plate on steel surfaces which interferes with adhesion of the intended plate.
- Avoid tin-lead soldering for parts that will be plated. Use silver solder instead. There are no good conventional procedures to plate over lead. In addition, lead contaminants in plating baths add significantly to platers’ expense and difficulty of waste management.
- Mask the stainless steel or difficult to plate components and plate only the steel. Realize this process can still cause slight flaking at the weld joints if the masking is not perfect.
- Discuss unavoidable combinations of dissimilar metals with your plater.
- Use cold-rolled steel whenever possible in assemblies to be plated. Hot-rolled steels have carbon scales on their surfaces which interfere with plating bonds.
- Clean surfaces before welding if hot-rolled steel must be used. Welding hot-rolled steel increases the difficulty of removing scale. Clean surfaces by abrasive blasting or immersion in a strong acid pickle.
- Avoid mixing metal parts in an assembly that will be plated or chemically finished. The presence of more than one metal can create a corrosion cell within a part which has been known to completely destroy components by treating them as sacrificial anodes. When significant destruction does not occur staining is quite common.
- Assemble parts after plating if the use of dissimilar metal components is unavoidable. Join with screws, bolts, rivets or press-fit, as appropriate.
- Avoid welding stainless steel and steel components together. To prevent flaking or peeling of the plating, coat the whole assembly with a nickel strike before plating if the use of both is unavoidable.
- Consider polishing as an alternative to electroplating stainless steel assemblies.
Other Examples of Problem Combinations
- Mixed aluminum alloys being anodized are susceptible to sacrificial corrosion. Alloys 6061 and 2024 (the anode) are an extreme case where the 2024 part is likely to be destroyed during anodizing.
- Steel inserts, and most other types of metal inserts, in aluminum parts being anodized can be severely etched. The heat generated by the acid attacking the insert can burn the surrounding aluminum. Titanium is the only metal which is safe in the presence of aluminum during anodizing.
- Stainless pemnuts, pop rivets and similar hardware attached to steel parts undergoing zinc plating are a common problem in which plating will not adhere to the stainless steel without a nickel strike coating.
Get it Plated Right Series
- Introduction: Cleaning and Design for Plating
- Fact Sheet #1: Cleaning Processes – Capabilities and Limitations
- Fact Sheet #2: Particulate Contamination on Part Surfaces – Eliminate to Reduce Plating Rejects
- Fact Sheet #3: Selecting Materials for Plated Parts
- Fact Sheet #4: Dried on Process Fluids and Fluid Combinations
- Fact Sheet #5: High Temperature Processing Burns on Soils
- Fact Sheet #6: Holes, Seams, Threads, Recesses and Tubing Assemblies