PM Surface Treatment Selection Guide

Treatment Selection Matrix

May require pore sealing treatment *When used with oil

1. Steam Treatment (Black Oxide)

Process Description

Parts are exposed to superheated steam (typically 510-565 deg C for 30-90 minutes), forming a black magnetite (Fe3O4 oxide layer on the surface and in near-surface pores.

Properties

• Layer thickness: 1-3 um oxide
• Color: Blue-black to black
• Hardness change: Negligible
• Corrosion resistance: Good for mild environments
• Dimensional change: Typically <0.01mm

Advantages

• Low cost (typically $0.15-0.30 per part in batch processing)
• Seals surface porosity, improving oil retention
• can materially improve corrosion resistance over untreated PM
• No masking required (entire part treated uniformly)
• Well-suited to PM materials (oxide penetrates surface pores)

Limitations

• Limited protection in harsh corrosive environments
• Black color only (no color options)
• Not suitable for decorative applications
• Long-term outdoor exposure requires additional protection

Typical Applications

• Oil-lubricated automotive gears
• Industrial machinery components
• Interior environment applications

Cost Range

$0.15-0.30 per part (batch process, volume-dependent)

2. Zinc Plating

Process Description

Electroplating or mechanical plating deposits zinc layer on part surface. PM parts typically require pore sealing (resin impregnation or copper infiltration) before plating.

Properties

• Thickness: Typically 5-15 um
• Color: Bright silver (various chromate finishes available)
• Corrosion mechanism: Sacrificial protection
• Salt spray resistance: 200-500 hours per ASTM B117

Advantages

• Excellent corrosion protection through sacrificial mechanism
• Cost-effective compared to other platings
• Multiple finish options (bright, matte, colored chromate)
• Well-established industrial process

PM-Specific Considerations

• Critical: Porosity can trap plating solutions, causing hydrogen embrittlement risk
• Typically requires pre-sealing treatment (resin impregnation)
• Thickness build-up in recesses may affect dimensions (+/-0.01mm typical)

Typical Applications

• Outdoor hardware and fasteners
• Automotive components
• Appliance parts

Cost Range

$0.50-1.50 per part (depends on size; includes sealing treatment)

Important: Always specify to plating vendor that parts are PM and require appropriate pre-treatment.

3. Powder Coating

Process Description

Electrostatic spray application of powder paint (epoxy, polyester, or polyurethane based), followed by curing at 160-200 deg C for 10-20 minutes.

Properties

• Thickness: 60-150 um (relatively thick coating)
• Color options: Virtually unlimited
• Finish: Matte, semi-gloss, or gloss
• Corrosion resistance: Excellent (barrier coating)

Advantages

• Excellent corrosion resistance
• Wide color and finish selection
• Environmentally friendly (no VOCs)
• Thick coating can cover minor surface imperfections
• Generally good adhesion to PM substrates

Limitations

• Thick coating may affect tight dimensional tolerances
• Temperature limit typically 150 deg C maximum service temperature
• Susceptible to chipping from mechanical damage

Typical Applications

• Consumer products (appliances, furniture)
• Outdoor equipment
• Decorative applications

Cost Range

$1.00-3.00 per part (varies with part size and color complexity)

4. Nitriding (Ferritic Nitrocarburizing)

Process Description

Parts heated in nitrogen-rich atmosphere (550-590 deg C for 1-4 hours), with nitrogen diffusing into surface to form hard nitride compounds.

Properties

• Case depth: 10-200 um (time-dependent)
• Surface hardness: 60-70 HRC (iron nitride layer)
• Core hardness: Unchanged
• Color: Matte gray to black
• Dimensional change: Negligible (low-temperature process)

Advantages

• Very high surface hardness and wear resistance
• Improved fatigue strength
• Low-temperature process minimizes distortion
• Works directly on PM parts without sealing
• Enhances surface properties without affecting core

Limitations

• Relatively expensive process
• Not decorative (gray/black matte finish)
• Limited intrinsic corrosion resistance (post-oxidation step often added)

Typical Applications

• High-wear gears (automotive, industrial)
• Valve components and sealing surfaces
• Tools and forming dies
• Wear-critical sliding components

Cost Range

$3.00-8.00 per part (depends on required case depth)

5. Resin Impregnation (Pore Sealing)

Process Description

Parts submerged in liquid resin (anaerobic or thermal-cure types), with vacuum applied to draw resin into pores, then cured through heat or chemical reaction.

Purpose and Function

• Primary function: Seals porosity for leak-tight applications
• Secondary benefits: Prevents fluid absorption, enables subsequent plating
• Not a surface finish (typically invisible after curing)

Properties

• Fills pores in parts with up to 20% porosity
• No dimensional change
• No visible color change
• Temperature limit: 150-200 deg C (resin dependent)

Advantages

• Makes PM parts suitable for pressure applications
• Enables electroplating by preventing solution entrapment
• Improves machinability (resin lubricates cutting tools)
• Relatively quick process

Limitations

• Temperature service limitation (resin-dependent)
• Not suitable for high-temperature applications

Typical Applications

• Hydraulic and pneumatic components
• Pre-treatment for electroplating
• Parts requiring subsequent machining
• Leak-tight pressure vessels

Cost Range

$0.30-1.00 per part

6. Electrocoating (E-Coat)

Process Description

Parts immersed in electrically charged paint bath, with paint depositing on surface through electrodeposition, followed by curing at 160-190 deg C.

Properties

• Thickness: 15-30 um (uniform coating)
• Corrosion resistance: Excellent
• Color options: Limited (typically black, gray, or primers)
• Coverage: Excellent (reaches recesses and blind holes)

Advantages

• Uniform coating thickness even in complex geometries
• Excellent throwing power for hard-to-reach areas
• Automotive-grade corrosion protection
• Cost-effective for high volumes

Limitations

• Limited color palette
• May require pre-treatment for highly porous PM substrates

Typical Applications

• Automotive components
• Outdoor equipment requiring corrosion protection
• Primer coat for subsequent top coating

Cost Range

$0.80-2.00 per part (high-volume pricing)

7. Phosphate Coating

Process Description

Chemical conversion coating formed by immersing parts in phosphoric acid solution (typically 70-90 deg C), creating crystalline phosphate layer on surface.

Properties

• Thickness: 2-10 um crystalline structure
• Color: Gray to black
• Corrosion resistance: Fair (excellent when combined with oil)
• Wear resistance: Good in lubricated service

Advantages

• Improves break-in period for gears (reduces initial friction)
• Excellent oil retention due to porous coating structure
• Provides excellent base for paint adhesion
• Low cost relative to other treatments

Limitations

• Not decorative
• Requires oil film for full corrosion protection in service

Typical Applications

• Oil-lubricated gears and bearings
• Pre-treatment for painting or powder coating
• Threaded fasteners (with oil preservation)

Cost Range

$0.20-0.50 per part

Treatment Selection Decision Tree

Step 1: Identify Primary Requirement

Corrosion Protection Priority:

• Mild indoor environment -> Steam Treatment
• Outdoor or marine environment -> Zinc Plating or Powder Coating
• Long-term outdoor exposure -> E-Coating or Powder Coating

Wear Resistance Priority:

• Moderate wear with lubrication -> Phosphate + Oil or Steam Treatment
• High wear without lubrication -> Nitriding
• Extreme wear (sliding contact) -> Chrome Plating or Nitriding

Leak-Tight Requirement:

• Hydraulic or pneumatic application -> Resin Impregnation (mandatory)
• Then apply additional corrosion protection if needed

Decorative Appearance:

• Color requirement -> Powder Coating
• Bright metallic finish -> Nickel or Chrome Plating

Cost-Sensitive Application:

• Steam Treatment ($0.15-0.30)
• Phosphate Coating ($0.20-0.50)

Step 2: Consider PM-Specific Requirements

High Porosity Parts (>15% porosity):

• Resin impregnation may be required before plating
• Consider infiltration for critical applications

Tight Dimensional Tolerances:

• Prefer thin treatments (steam, phosphate, nitriding)
• Account for coating thickness if using powder coating or thick plating

Complex Geometries:

• E-coating offers excellent throwing power
• Powder coating or plating may have coverage limitations

Application-Specific Recommendations

Multiple Treatment Combinations

Some applications benefit from combination treatments:

Common Combinations:

1. Resin Impregnation + Zinc Plating (leak-tight + corrosion)
2. Steam Treatment + Oil Impregnation (gears, bearings)
3. Nitriding + Post-Oxidation (wear + corrosion)
4. Phosphate + Powder Coating (paint adhesion)

Get Surface Treatment Recommendations

SinterWorks provides surface treatment consultation:

• Application-specific treatment selection
• Cost-performance optimization analysis
• In-house capabilities: Steam treatment, resin impregnation
• Partner network for plating, coating, and nitriding services

Contact us for treatment selection guidance.