Can Powder Metallurgy Parts Be Plated or Coated?

# Can Powder Metallurgy Parts Be Plated or Coated?

Yes-powder metallurgy parts can be plated and coated, and surface finishing is routinely applied to PM components for corrosion protection, wear resistance, appearance, and functional requirements. But the porous surface of a sintered part changes how many finishing processes behave, and not every plating method that works on wrought steel works equally well on PM.

This guide covers the most common surface finishing options for PM parts, what preparation is required, and where porous surfaces create complications that need to be managed.

Why PM Surface Finishing Is Different

The central difference between finishing a PM part and finishing a wrought or machined part is open surface porosity. Most PM parts (standard structural grades) have 5-15% porosity. At the surface, this means:

• Pores open to the surface can trap plating solution, rinse water, and process chemicals
• Trapped liquids bleed out over time (hours, days, or weeks), causing staining, blistering, or adhesion failure beneath the coating
• Surface roughness is higher than machined metal (Ra 0.8-3.2 um typical as-sintered), affecting deposit uniformity
• Oil-impregnated PM parts have oil on and near the surface, which must be removed before plating

These issues do not prevent plating-but they require specific preparation steps that would not be needed for solid metal.

Step One: Seal the Pores Before Plating

For any wet plating process (electroplating, electroless plating), the most reliable approach is to seal the surface pores before plating. The standard method is resin impregnation.

Resin impregnation fills the interconnected pore network with a cured thermosetting resin (typically anaerobic acrylate). After impregnation and cure:

• Surface pores are sealed; no solution can enter or bleed out
• The part behaves like a solid metal surface for subsequent plating steps
• Plating adhesion, corrosion resistance, and appearance are significantly improved

Resin impregnation before plating is a common practice for PM parts going through electroplating or electroless plating. Skipping this step is the most common cause of blister, stain, and adhesion failures on plated PM parts.

For oil-impregnated PM parts that require plating, the oil must be fully removed (solvent cleaning, baking, or both) before resin impregnation. Residual oil prevents resin from filling pores properly.

Common Plating and Coating Processes for PM

Electroless Nickel

Electroless nickel (EN) is one of the most widely used finishes on PM parts. It deposits uniform nickel-phosphorus alloy on all surfaces-including internal pores and recesses-without requiring electrical contact. Key properties:

• Deposit thickness: 5-25 um typical
• Corrosion resistance: representative ranges around 200-1000+ hours salt spray depending on thickness and P content
• Hardness: 45-65 HRC as-plated; up to 68-70 HRC after heat treatment
• Uniform coverage on complex geometry, including internal features

Electroless nickel on resin-impregnated PM provides excellent corrosion protection and is used on PM parts in automotive, industrial, and fluid system applications.

Preparation: Resin impregnation -> clean and activate -> EN plate

Zinc Electroplating

Zinc electroplating is a cost-effective corrosion protection finish widely used on iron-based PM structural parts. Typical applications: fasteners, brackets, structural housings.

• Deposit thickness: 5-25 um typical
• Corrosion resistance: representative ranges around 72-240+ hours salt spray (with passivate/chromate)
• Cost: lower than EN; very high-volume process
• Risk: acid zinc plating baths can attack PM surfaces if pores are not sealed

For zinc plating on PM, resin impregnation is recommended, especially for alkaline zinc processes. Acid zinc is harder on PM surfaces.

Alternative without impregnation: Mechanical zinc plating (barrel blasting with zinc powder and impactors) deposits zinc without liquid immersion, avoiding trap-and-bleed issues. It is used for PM fasteners and small hardware when impregnation is not specified.

Zinc-Nickel Electroplating

Zinc-nickel alloy plating (12-16% Ni) provides significantly better corrosion resistance than pure zinc-often in the 500-1000 hour salt spray range with passivate. Used in automotive and industrial applications where higher corrosion protection is required without the cost of electroless nickel.

Same preparation considerations as zinc electroplating apply.

Chrome Plating (Hard Chrome and Decorative)

Hard chrome plating can be applied to PM parts for wear and corrosion resistance, but it is challenging on porous substrates:

• Hydrogen embrittlement during plating is a concern for higher-strength PM grades
• The porous surface requires thorough preparation (impregnation) to prevent hydrogen trapping and subsequent blistering
• Decorative chrome (thin, on nickel undercoat) is more manageable than thick hard chrome

Hard chrome on PM is not a common production process and should be evaluated carefully with the plating supplier. Alternatives like electroless nickel or PVD coatings are often preferred for wear-critical PM surfaces.

Copper Plating

Copper plating is used as an undercoat for multi-layer systems (copper-nickel-chrome) and can be applied to PM. Copper is relatively forgiving of surface porosity and is a common first strike plate to seal the surface before subsequent layers.

Phosphate Coating (Zinc or Manganese Phosphate)

Phosphate conversion coatings are widely used on iron-based PM parts. They are applied by immersion in phosphate solution and create a crystalline layer that:

• Provides moderate corrosion protection (typically 24-72 hours salt spray alone)
• Improves oil and paint adhesion
• Adds break-in lubrication for gears and sliding parts

Manganese phosphate + oil is a common finish for PM gears, cams, and structural components in industrial applications. Zinc phosphate is used where paint adhesion is needed.

Phosphate coating enters the surface pores by design-this is not a problem because phosphate solution does not trap and bleed the same way electroplating solutions do. Phosphate + oil seals pores and provides adequate surface protection for many indoor mechanical applications.

Steam Treatment + Oil

Steam treatment (Fe3O4 black oxide) followed by oil impregnation is a common and cost-effective finish for iron-based PM parts. It provides moderate corrosion protection, often in the 50-150 hour salt spray range depending on oil and geometry and leaves no dimensional buildup. It is not technically "plating" but functions as a surface finish in many applications.

Powder Coating and Paint

Powder coating and liquid paint can be applied to PM parts, but pore sealing before coating is even more important than for electroplating. If surface pores are not sealed:

• Outgassing during the powder coat cure cycle (120-200 deg C) creates bubbles and craters in the coating
• Paint adhesion is poor on open-pore surfaces

Resin impregnation before powder coating eliminates outgassing. Phosphate pretreatment improves adhesion.

PVD and CVD Coatings

Physical vapor deposition (PVD) coatings-TiN, TiAlN, CrN, DLC (diamond-like carbon)-can be applied to PM parts for wear and friction reduction. These are dry, vacuum-based processes that do not involve liquid immersion, avoiding the trap-and-bleed issue.

PVD requires high surface density and cleanliness to achieve good adhesion. Parts are typically sized or ground before PVD to provide a smooth, dense surface. Resin impregnation before PVD is recommended for standard-density PM parts.

PVD-coated PM tooling inserts and wear components are produced in specialty PM grades for cutting tools and forming applications.

Special Case: Stainless Steel PM

316L and 304 stainless PM parts are often finished without plating-the passive film on the stainless surface provides adequate corrosion resistance. Where additional finish is required:

• Electropolishing is the standard finishing operation for food-grade and medical-adjacent stainless PM parts. It smooths the surface, removes free iron, and improves the passive film without adding a deposited layer.
• Passivation (citric or nitric acid) removes surface contamination and strengthens the passive film.
• Electroless nickel can be applied to stainless PM for specific functional requirements, but adhesion requires activation with an appropriate strike.

Finishing Process Selection Guide

Drawing Specification Notes

When specifying surface treatment on a PM drawing:

1. Always specify impregnation requirement separately. "Zinc electroplate per ASTM B633" does not automatically include resin impregnation. If you require pore sealing before plating, specify it explicitly: "Resin impregnate (MIL-I-17563 Class A or equivalent) prior to plating."
2. Specify coating thickness and acceptance class. Plating standards (ASTM B633, ASTM B733, etc.) define thickness classes-match the class to your corrosion or wear requirement.
3. Confirm with the plating supplier. Not all plating shops have experience with PM substrates. A plating supplier experienced with sintered parts will handle activation, rinse cycles, and inspection differently than one accustomed to wrought steel only.

Contact us if you have questions about specifying surface treatment for your PM part. We can advise on preparation, coating selection, and drawing callouts based on your application and environment.