Metal Surface Treatment, Conversion Coatings and Plating: Magnesium Anodic Treatment

Magnesium Anodic Treatment — MIL-M-45202C

The HAE anodic finish is probably the hardest coating currently available for magnesium. It exhibits stability at high temperatures, good dielectric strength, and serves as an excellent paint base. Requires resin seal or paint for maximum corrosion protection.

Tin-Lead — MIL-P-81728A

Excellent solderability. Matte or bright luster acceptable. For electronics components, only matte or flow-brightened finish.

• Brightened electronics components: maximum thickness 0.0003 in.
• Tin 50–70% by weight, lead remainder: 0.0003–0.0005 in.

Hard Facing: When the Surface Must Be Harder Than the Part

the practitioner's breakthrough came when a senior metallurgist showed her a set of agricultural crusher jaws that had been hard-faced with cobalt-chromium alloy. The jaws had survived three seasons of crushing rock — while uncoated jaws lasted one.

Hard facing is a method of adding a coating, edge, or point of a metal or alloy capable of resisting abrasion, corrosion, heat, or impact to a metal component. It can be applied equally well to new parts or worn parts.

Application methods:

• Oxyacetylene gas welding
• Shielded-metal arc welding (SMAW)
• Submerged arc welding
• Plasma arc welding
• Inert-gas-shielded arc welding (consumable and nonconsumable electrode)
• Spraying process (wire or powder)

Selection principle: The first consideration is always the type of service the part will undergo. Other factors include machinability, cost, porosity, appearance, and ease of application. Generally, the greater the hardness, the greater the resistance to abrasion and impact wear.

Hard-Facing Material Comparison

Detailed NiCr Properties (Rod vs. Electrode)

The hardness difference between rod and electrode deposits matters in specification:

Lower hardness values and greater ranges in electrode deposits are attributed to dilution of deposit and base metals.

Hot hardness — NiCr-C electrode deposits: Range from Rc 49 (instantaneous) to Rc 31 (3-minute loading) over 600–1000°F. Rod deposits: Rc 55 to Rc 40 under the same conditions.

Compression properties (all NiCr alloys):

• Elastic limit: 42,000 lb/in²
• Yield strength at 0.01% offset: 92,000 lb/in²
• Yield strength at 0.10% offset: 150,000 lb/in²
• Yield strength at 0.20% offset: 210,000 lb/in²

Flame Spraying: Building Surfaces from Molten Particles

The flame spraying process deposits metals, alloys, ceramics, and cermets on metallic or other surfaces. Applications include:

• Building up worn or undersize parts
• Providing wear-resisting or corrosion-resisting surfaces
• Correcting defective castings

Three primary equipment types:

. Wire-Fed Combustion Spray

Wire is fed automatically through the spray gun nozzle. A combustible gas (usually acetylene), oxygen, and compressed air melt and atomize the wire, blowing it against the surface.

Key capability: Any desired thickness can be deposited. Materials include steels (low to high carbon), brass and bronze compositions, babbitt metal, tin, zinc, lead, nickel, copper, and aluminum.

Production applications: Automotive exhaust valves, transfer ink rollers, truck clutch plates, glass meter box windows, aluminum-on-cloth for electrolytic condenser plates, zinc/copper on ceramic insulators.

. Powder-Fed Combustion Spray

Uses metal, refractory, and ceramic powder instead of wire. Employs two gases: oxygen and a fuel gas (usually acetylene; sometimes hydrogen).

Four basic coating powder types:

1. Ceramics
2. Oxidation-resistant metals and alloys
3. Self-bonding alloys
4. Alloys for fused coatings

These produce wear-resistant, corrosion-resistant, heat-resistant, and electrically conductive coatings.

. Plasma Flame Spraying

Uses plasma to raise material vapors to a higher energy level than ordinary gases. This raises the temperature ceiling and provides a controlled atmosphere using inert or chemically inactive gas, controlling oxidation during heating and application.

Plasma Arc Surface Coating

A transferred arc (between electrode and workpiece) applies coatings to the workpiece surface. Key parameters:

• Arc temperatures: 25,000–50,000°F (14,000–28,000°C)
• Deposition rates: Up to 15 lb/h (powder) or 28 lb/h (wire)
• Base metal dilution: Can be held below 5% if required

The nontransferred arc process (arc between electrode and nozzle, not touching the work) is used for metal spraying — building surfaces for hard facing and applying anticorrosion/barrier layers. Over 500 different powder combinations are available, with deposition rates up to 100 lb/h.

Laser Surface Treatment: Precision Without Distortion

This was the practitioner's "aha" moment. When she learned that a defocused CO₂ laser impinging on room-temperature steel has over 90% of its power reflected (about 93% for steels), she understood why laser heat treatment requires a completely different approach than conventional heating.

The Reflectivity Problem

Laser heat treatment operates at power densities below 10⁴ W/cm² — insufficient to overcome reflectivity effects. The surface must be prepared to enhance absorption by one of several methods:

• Surface roughening — creates tiny craters that trap beam energy long enough to raise surface temperature to where more energy is absorbed
• Surface coating — black enamel paint is common; the laser vaporizes the enamel, leaving a clean surface beneath

Once absorbed, the laser energy (converted to heat) raises the temperature in the beam pattern. Dwell time controls the depth of heat treatment and is extremely effective for case depth control in hardening.

Materials Applicability

Microstructure effect on case depth:

Typical hardening rate: 130 cm²/min (20 in²/min) for a 1 mm (0.039 in.) case depth in 4140 steel.

Compared to conventional techniques: Laser hardening is typically slower than induction heating. However, by limiting the area to be hardened, the laser eliminates residual heat effects that cause part distortion — making it cost-effective for precision components.

Laser Cladding

In laser cladding, a shaped or defocused beam heats preplaced or gravity-fed powdered alloys. The cladding alloy melts, flows across the substrate surface, and rapidly solidifies when laser power is removed.

Compatible alloys include: Stellites, Colmonoys, alloys containing carbides, Inconel, Triballoy, Fe-Cr-C-X alloys, tungsten and titanium carbides.

The key advantage — minimal dilution:

The laser process produces a dense, homogeneous, nonporous clad layer that is metallurgically bonded to the substrate — in contrast to the mechanically bonded, more porous layer produced by other methods.

Laser Marking: Permanent Identification Without Contact

Two groups of laser marking technology:

Mask Marking

A CO₂ laser beam is projected through a reflective mask, reimaged by optics onto the material's surface. The absorbed heat changes the molecular structure to produce a visible mark.

• Line speeds: Up to 20,000 marks/h
• Energy densities: 1–20 J/cm²
• Minimum line width: 0.1 mm (0.004 in.)
• Applications: Clouding PVC or acrylics, changing colored surfaces by adjusting pigment proportions, ablating surface layers to expose sublayers of different color

Scanned-Beam Marking

A pulsed YAG or CO₂ laser beam is directed onto the surface by controlled mirror oscillation in a preprogrammed pattern. Provides virtually unlimited choice of patterns.

• Heat-type marks: Up to 2500 mm/s (100 in/s)
• Engraved marks: 500–800 mm/s (20–30 in/s)
• Typical writing field: 100 × 100 mm (4 × 4 in.)

For highly reflective metals like aluminum, better results come from pretreating the surface (e.g., anodizing). Not all scanned-beam applications remove base metal — some remove only a coating or produce a heat discoloration.

Act IV: The Etching Workshop — Chemical Surface Artistry

Etching Fluids for Different Metals

The final piece of the practitioner's surface treatment education came from the company's in-house toolroom, where master toolmakers still used acid etching for precision scale graduation and identification marking.

Standard etching method for steel: Apply a thin, even coating of beeswax or similar acid-resistant substance (the "resist"), scribe lines through the wax to expose the steel, then apply acid.

Resist composition (for fine graduation work): Approximately 50% asphaltum, 25% beeswax, plus small percentages of Burgundy pitch, black pitch, and turpentine. For some work, melted paraffin is poured over the surface at elevated temperature to form a thin protective coating.

Etching fluid reference:

Act V: The Heat-Affected Zone — Understanding What Heat Treatment Does to Surfaces

the practitioner's final lesson came from the EDM shop, where wire-cut parts showed surface damage that was invisible to the naked eye but devastating in service.

The Recast Layer and HAZ in EDM

When EDM sparks strike the workpiece, the dielectric oil breaks down into hydrocarbons, tars, and resins. The molten metal draws out carbon atoms and traps them in the resolidified metal, forming a very thin, hard, brittle recast layer covering the heat-affected zone (HAZ).

Recast layer characteristics:

• Appearance: White
• Composition: Particles melted by sparks, enriched with carbon, drawn back to surface
• Hardness: Harder than parent metal — can be as hard as glass
• Required action: Must be reduced or removed by vapor blasting with glass beads, polishing, electrochemical or abrasive flow machining — or risk cracking and flaking in service

HAZ characteristics (in steel):

• Consists of martensite hardened by heating/cooling sequences
• Expansion and contraction rates differ from parent metal
• Under thermal cycling, the differential stresses may cause surface cracks
• HAZ depth depends on amperage and on-time: increases to about 0.012–0.015 in. deep
• Residual stress can reach 650 N/mm²

Best practice: Program successive cuts so that each cut removes most of the HAZ from the previous cut. Reduce cut depth gradually until finishing cuts produce HAZ thickness < 0.0001 in.

Heat Treatment of Steel Castings

Steel castings can be heat treated to achieve:

• Diffusion of carbon or alloying elements
• Softening or hardening
• Stress relieving
• Toughening
• Improved machinability
• Increased wear resistance
• Removal of hydrogen entrapped at the casting surface

Heat treatment of steel castings follows closely that of wrought steel of similar composition.

Mechanical properties achievable by heat treatment:

The Decision Framework: Choosing the Right Surface Treatment

the practitioner built this framework after months of investigation, testing, and failure analysis. It became the standard reference in her plant.

Surface Treatment Selection Matrix

The Hydrogen Embrittlement Decision Tree

This is the issue that catches engineers off guard and causes catastrophic failures:

Is the steel ≥ Rc 33 (or ≥ 1000 MPa tensile)?
├── YES → Hydrogen embrittlement risk exists
│   ├── Can you avoid electroplating entirely?
│   │   ├── YES → Use vacuum cadmium, mechanical plating, or conversion coatings
│   │   └── NO → Proceed with extreme caution:
│   │       ├── Stress relieve BEFORE plating (375°F, 3+ hours)
│   │       ├── Bake AFTER plating (375°F, 3–24 hours depending on process)
│   │       ├── Bake within 4 hours of plating completion
│   │       └── Do NOT flex springs or flexure parts before baking
│   └── Is the steel ≥ Rc 40 (or ≥ 1700 MPa tensile)?
│       ├── YES → Some processes are PROHIBITED
│       │   ├── Zinc plating: SHALL NOT be applied > 1700 MPa
│       │   ├── Nickel plating: Requires specific agency approval > 220,000 psi
│       │   └── Vacuum cadmium: Recommended alternative for ≥ 220,000 psi
│       └── Extended bake times required (8–12+ hours for chrome)
└── NO → Standard plating procedures apply

Engineering takeaway

the practitioner's six-month journey from catastrophic die failure to surface treatment expertise taught her a principle that every engineer, machinist, and manufacturing professional should internalize:

The surface is not separate from the part. It IS the part — at the one place where the part meets the world.

Every coating, plating, conversion treatment, and surface engineering process described in this guide exists because someone, somewhere, learned the hard way that naked metal isn't enough. Corrosion doesn't care about your alloy selection if the surface isn't protected. Wear doesn't care about your heat treatment if the contact interface isn't engineered. Fatigue doesn't care about your design margins if the surface finish initiates cracks.

The specifications in this guide — the MIL-SPECs, the ASTM standards, the AMS requirements — aren't bureaucratic red tape. They're the accumulated wisdom of millions of failures, distilled into precise instructions for preventing the next one.

Your Next Step

Look at the last part that failed in your shop, your plant, or your client's facility. Ask yourself:

Was the failure at the surface?

If the answer is yes — and statistically, it probably is — then the right surface treatment could have prevented it. Go back through this guide with that specific failure in mind. Match the failure mode to the treatment category. Check the specification requirements against what was actually applied.

The answer is almost always in the surface.

Quick Reference: Military and Industry Specifications Covered

This guide covers conversion coatings, electroplating, electroless plating, anodizing, hard facing, flame spraying, plasma arc coating, laser heat treatment, laser cladding, laser marking, etching, coloring, passivation, and all associated military and industry specifications. Bookmark it. You'll be back.