Electroplating
Electrochemical deposition of a metal layer onto a substrate. The most common surface treatment for steel parts -- used for corrosion protection, wear resistance, solderability, and appearance. This guide covers what to specify, what it costs, what can go wrong, and when hydrogen embrittlement will kill your parts.
Which Plating Do You Need?
Start with your primary requirement. Most steel parts get zinc plating -- it is the cheapest option that works. Only move to more expensive plating when you have a specific reason. Use this table to decide.
| If Your Part Needs... | Plating | Typical Thickness | Cost Factor |
|---|
| Basic corrosion protection on steel (indoor or sheltered outdoor) | Zinc (clear chromate) | 5–8 μm | 1x (baseline) |
| Outdoor corrosion protection (automotive, construction) | Zinc (yellow or olive drab) | 8–15 μm | 1x |
| High corrosion resistance with RoHS compliance (automotive, European market) | Zinc-nickel alloy | 8–15 μm | 2–3x |
| Hard, wear-resistant surface on steel | Hard chrome | 10–250 μm | 3–5x |
| Bright, decorative finish (consumer products, fixtures) | Nickel + decorative chrome | Ni: 10–20 μm / Cr: 0.25 μm | 2–4x |
| Uniform coating on complex geometry (no current needed) | Electroless nickel (ENP) | 10–50 μm | 3–5x |
| Solderability (electrical contacts, PCB terminals) | Tin or tin-lead | 5–15 μm | 2–3x |
| Electrical conductivity with corrosion protection | Tin or silver | 5–15 μm | 2–4x |
| Pre-treatment before painting or adhesive bonding | Copper (strike layer) | 2–5 μm | 1–2x |
| Maximum corrosion resistance, marine environment (and cost is no object) | Cadmium | 8–15 μm | 4–6x |
Rule of Thumb
If you do not know what plating to use and the part is steel, specify zinc plating with yellow chromate. It covers 80% of use cases at the lowest cost. Move to zinc-nickel only if the customer requires RoHS compliance and 500+ hours salt spray. Move to hard chrome only if there is a real wear requirement.
Plating Types at a Glance
| Property | Zinc | Zinc-Nickel | Nickel (electro) | Electroless Ni | Hard Chrome | Decorative Chrome | Tin | Copper |
|---|
| Thickness Range | 5–25 μm | 8–15 μm | 5–50 μm | 10–75 μm | 10–500 μm | 0.25–0.5 μm | 5–15 μm | 2–30 μm |
| Hardness (HV) | 70–120 | 350–500 | 150–300 | 500–700 | 800–1000 | 900–1100 | 10–20 | 50–100 |
| Corrosion (salt spray hrs) | 96–500 | 500–1000 | 48–200 | 200–500 | 24–200 | 48–96 | 48–200 | 24–96 |
| Cost Factor | 1x | 2–3x | 2–3x | 3–5x | 3–5x | 2–4x | 2–3x | 1–2x |
| Wear Resistance | Low | Moderate | Moderate | Good | Excellent | Low (thin) | Very low | Low |
| Typical Use | Fasteners, brackets, stampings | Auto fasteners, Euro market parts | Decorative, dimensional restore | Complex shapes, valves, pump parts | Hydraulic rods, molds, wear surfaces | Consumer trim, faucets, handles | Electrical contacts, terminals, PCB pins | Pre-plate layer, electrical grounding |
| H-Embrittlement Risk | Yes | Yes | Yes | Low | High | Moderate | Low | Low |
Zinc Plating Variants
Zinc is the workhorse plating for steel. It corrodes sacrificially -- the zinc layer rusts instead of the steel underneath. The corrosion resistance is determined primarily by the chromate (passivation) film applied on top of the zinc. Chromate chemistry matters more than zinc thickness for salt spray performance.
| Chromate Type | Appearance | Salt Spray (hrs) | RoHS Compliant | Typical Application |
|---|
| Clear / Blue (Cr3+) | Clear to slight blue iridescence | 96–120 | Yes | Indoor, consumer goods, electronics hardware |
| Yellow Iridescent (Cr3+) | Iridescent yellow, sometimes rainbow | 120–200 | Yes | Outdoor, automotive brackets, general industrial |
| Yellow Iridescent (Cr6+) | Bright iridescent yellow | 150–250 | No | Military, aerospace (RoHS-exempt), legacy auto |
| Olive Drab (Cr6+) | Dark olive green | 200–500 | No | Military hardware (MIL-DTL-5541 equivalent for zinc) |
| Black (Cr3+) | Matte black | 96–200 | Yes | Fasteners, appearance parts, automotive interior |
| Sealer / Topcoat (over Cr3+) | Depends on base chromate | Adds 200–500 hrs | Yes | When Cr6+ salt spray performance is needed without Cr6+. Common for European automotive. |
Trivalent vs Hexavalent Chromium
Cr6+ (hexavalent) gives better salt spray performance and was the industry standard for decades. RoHS and REACH regulations restrict Cr6+ in most consumer and automotive products sold into the EU. Trivalent (Cr3+) chromates are RoHS-compliant but deliver lower salt spray hours on their own. The workaround: apply a Cr3+ chromate plus an organic or inorganic sealer topcoat. This matches or exceeds old Cr6+ performance at higher cost.
Zinc-Nickel vs Zinc + Chromate
Zinc-nickel alloy plating (typically 12–15% Ni content) provides 5–8x the corrosion resistance of pure zinc with Cr3+ chromate -- without relying on Cr6+. It is the default for European automotive fasteners and parts requiring both high salt spray and RoHS compliance. The trade-off: it costs 2–3x more than plain zinc and hydrogen embrittlement risk is the same or slightly higher.
Hydrogen Embrittlement
This is the most dangerous failure mode in plated parts. Hydrogen atoms generated during the plating process can diffuse into the steel and make it brittle. The part may pass inspection and then crack suddenly under load -- often with no visible warning. Hardened and high-strength steels are most susceptible. This is a safety-critical topic. Get it wrong and parts break in service.
| Material / Condition | H-Embrittlement Risk | Baking Required? | Notes |
|---|
| Hardened steel (HRC 33+) | High | Mandatory | Any steel heat treated above ~HRC 33. Includes most fasteners above Grade 8 (metric 10.9), spring steel, bearing races. |
| High-strength low-alloy (HSLA) steel | High | Mandatory | 4140, 4340, 8620 after quench & temper. Check hardness -- if HRC 33+, bake. |
| Spring steel (music wire, 1095) | High | Mandatory | Springs are always high-risk. Even low-stress springs can fracture from hydrogen. Always bake. |
| Carbon steel (HRC 22–32) | Moderate | Recommended | Lower risk but still possible. Many OEMs require baking for any plated carbon steel regardless of hardness. |
| Mild steel (HRC below 22) | Low | Usually not required | Low-carbon steel, unhardened. Hydrogen diffuses out naturally. Baking is precautionary, not mandatory. |
| Stainless steel | Low | No | Austenitic SS (304, 316) is not susceptible. Martensitic SS (410, 420) can be -- treat like hardened steel. |
| Cast iron | Low | No | The graphite structure in cast iron does not trap hydrogen the way hardened martensite does. |
| Copper, brass, aluminum | None | No | These metals are not susceptible to hydrogen embrittlement. |
Baking Procedure
| Parameter | Specification | Notes |
|---|
| Temperature | 190–200 °C (375–390 °F) | Must reach temperature within 4 hours of plating (or per ASTM B850). Lower temperatures are less effective. |
| Time | 4–24 hours | Minimum 4 hours for most parts. 8–23 hours per ASTM F1940 for fasteners. Springs and critical safety parts: 23+ hours. Time starts when the part reaches temperature, not when the oven turns on. |
| Timing | Within 4 hours of plating | Hydrogen diffuses into the steel during plating and acid pickling. The longer you wait to bake, the deeper it penetrates and the harder it is to remove. ASTM B850 mandates baking within 4 hours. |
| Reference Standards | ASTM B850, ASTM F1940, MIL-STD-1500A | F1940 is specifically for hydrogen embrittlement testing of fasteners. B850 covers baking after plating. |
What Happens If You Skip Baking
The hydrogen stays trapped in the steel lattice. Under tensile load, the hydrogen migrates to stress concentration points (threads, fillets, notches) and causes intergranular cracking. The part fractures at loads well below its rated capacity -- often at 20–50% of the expected yield strength. There is no visible warning: no deformation, no discoloration, no surface crack before failure. For safety-critical parts (fasteners, springs, lifting hardware, pressure vessel components), skipping baking is a potential liability issue. Always bake.
Dimensional Impact
Unlike anodizing (which grows both inward and outward), electroplating deposits only on the surface. Every surface that contacts the plating bath gets thicker. This matters for threads, press fits, bearing journals, and any feature with tight tolerances.
| Plating Type | Typical Thickness | Per-Side Buildup | Total on a Diameter | Thread Pitch Dia Impact |
|---|
| Zinc (thin, 5 μm) | 5 μm | +5 μm | +10 μm (0.0004 in) | Negligible on class 6g/2A |
| Zinc (standard, 8–12 μm) | 8–12 μm | +8–12 μm | +16–24 μm (0.0006–0.001 in) | May cause tight go-gauge on 2A threads. Size threads before plate or use 2B nut. |
| Zinc (thick, 15–25 μm) | 15–25 μm | +15–25 μm | +30–50 μm (0.0012–0.002 in) | Significant. Must undersize threads or chase after plating. |
| Zinc-nickel (8–15 μm) | 8–15 μm | +8–15 μm | +16–30 μm | Same as zinc at equivalent thickness. Account for it. |
| Electroless nickel (25 μm) | 25 μm | +25 μm | +50 μm (0.002 in) | Significant. Pre-size bores, mask bearing journals. |
| Hard chrome (50 μm) | 50 μm | +50 μm | +100 μm (0.004 in) | Critical. Always grind after hard chrome. Chrome is deposited oversized and ground to final dimension. |
| Decorative chrome (0.25 μm) | 0.25 μm | negligible alone | Negligible | The nickel underlayer (10–20 μm) is what adds thickness. |
| Tin (8 μm) | 8 μm | +8 μm | +16 μm | Low hardness means tin can deform in threads -- gauge carefully. |
Hard Chrome Grinding
Hard chrome is almost never used at the as-plated dimension. The standard process: plate to 50–100 μm oversize, then grind to final dimension. This gives you precise dimensional control and removes surface nodules. If you need hard chrome on a shaft, specify "HARD CHROME PLATE 0.002 IN MIN, GRIND TO FINAL DIMENSION" on the drawing.
Material Compatibility
Not every substrate plates well. Carbon steel is the easiest. Aluminum, titanium, and cast iron require special pre-treatment. Plating onto the wrong material without proper preparation results in poor adhesion, blistering, or no plating at all.
| Substrate | Zinc | Nickel | Chrome | Tin | Copper | Notes |
|---|
| Carbon steel (1018, 1045, A36) | Best choice | Good | Good | Good | Good | Standard substrate. Acid pickles clean, plates directly. No special treatment needed. |
| Alloy steel (4140, 4340, 8620) | Good | Good | Good | Good | Good | Same as carbon steel but watch hydrogen embrittlement. Bake after plating. |
| Stainless steel (304, 316) | Rarely | Good | Good | With strike | With strike | SS passive layer resists plating. Requires a Woods nickel strike or sulfamate strike to activate the surface before plating. |
| Copper / Brass | Possible | Good | Good | Good | Good | Zinc on copper has poor adhesion. Use nickel barrier layer first. Copper plates directly. |
| Aluminum | No | Zincate pre-treat | Zincate pre-treat | Zincate pre-treat | Zincate pre-treat | Aluminum forms an oxide instantly in air. Must use a zincate dip (double zincate for best results) to create a surface that accepts plating. Adhesion is the main failure point. |
| Cast iron (gray, ductile) | Difficult | Difficult | Difficult | Difficult | Difficult | High carbon and graphite content causes uneven plating, pitting, and poor adhesion. Surface porosity traps plating solution. Extended acid pickling and special copper strike needed. Not all shops will plate cast iron. |
| Titanium | No | Special process | Special process | Special process | Special process | Titanium has an extremely stable oxide layer. Requires proprietary pre-treatment (e.g., fluoride activation). Very few shops offer titanium plating. Usually not worth it -- consider anodizing titanium instead. |
| Zinc die cast (Zamak) | Cyanide zinc only | With copper strike | With copper strike | With copper strike | Good | Acidic zinc plating solution attacks the zinc die cast substrate. Must use alkaline (cyanide or non-cyanide) zinc bath. Always use a copper strike first for nickel/chrome. |
Aluminum Plating Reliability
Plating onto aluminum works but adhesion is the chronic problem. Even with double zincate treatment, the bond between aluminum substrate and plated layer is weaker than on steel. Thermal cycling and impact can cause delamination. If the plated aluminum part will see temperature swings or mechanical shock, validate adhesion with ASTM B571 (qualitative) or ASTM B533 (quantitative) testing before committing to production.
Cost Drivers
Plating pricing varies by region, shop size, and volume. But the relative cost structure is consistent. Here is what actually moves the price per part.
| Cost Factor | Impact | Detail |
|---|
| Setup / Lot Charge | High for small orders | Most shops charge a minimum lot fee ($30–150). On a 10-piece order, setup dominates the per-part cost. At 500+ pieces, the per-part plating cost drops significantly. |
| Plating Material | Moderate | Zinc is cheapest. Tin and copper are moderate. Nickel, chrome, and zinc-nickel are expensive -- the bath chemistry and metal anodes cost more. |
| Thickness | Moderate | Longer plating time = higher cost. Standard zinc (5–8 μm) is one price. Requesting 25 μm zinc adds cycle time and may push the lot charge higher. |
| Masking | $1–10 per part | Each masked feature (thread, bore, surface) requires labor. Simple tape mask: $1–3. Precision plug mask for a bore: $5–10. Multiple masked features add up fast. |
| Baking (H-embrittlement) | +$0.50–3 per part | Extra oven cycle (4–23 hours at 190 °C). Adds labor, energy, and delays shipment. Mandatory for hardened parts -- not optional. |
| RoHS Compliance | +10–30% | Trivalent chemistries cost more than hexavalent. Testing and documentation add overhead. Most European and consumer-product customers require it. |
| Military / Aerospace Spec | +20–50% | MIL-DTL-5541, AMS 2410, QQ-P-416, or ASTM B633 compliance adds process control, testing, and documentation. Lot traceability requirements drive up cost. |
| Rush / Expedite | +25–100% | Standard plating lead time: 3–7 working days. Rush disrupts batch scheduling. Some shops refuse rush orders entirely. |
| Part Size / Weight | Low to moderate | Very small parts need special racking (barrel plating helps). Very large parts may not fit standard tanks and need custom racks or manual processing. |
Common Mistakes
| Mistake | Consequence | Fix |
|---|
| Not specifying baking for hardened steel parts | Delayed cracking under load. Parts pass outgoing inspection but fail in service. Potential safety liability if the part is structural or load-bearing. | Add "BAKE PER ASTM B850, 190 °C MIN, 4 HR MIN WITHIN 4 HR OF PLATING" to the drawing. This is non-negotiable for HRC 33+ steel. |
| Not accounting for plating thickness on threads | Plated bolts do not thread into nuts. Go/no-go gauge fails. Threads feel tight or seize. Worst on thin plated parts (class 3B threads). | For zinc at 8–12 μm, account for 0.0003–0.0005 in per side on the pitch diameter. Oversize internal threads or undersize external threads before plating. Or specify post-plate thread chasing. |
| Specifying zinc plating on aluminum | Zinc plating bath (acidic) dissolves the aluminum substrate. Parts come out damaged, pitted, or with no plating at all. | Use anodize or conversion coating for aluminum corrosion protection. If plating onto aluminum is required, specify electroless nickel with zincate pre-treatment. |
| Calling out Cr6+ chromate for RoHS-restricted parts | Parts fail incoming inspection at the customer. Entire batch rejected. Expensive rework or scrap. | Check customer requirements first. If RoHS applies, specify trivalent (Cr3+) chromate with sealer topcoat to hit salt spray targets. |
| Specifying hard chrome without post-plate grinding | Surface is rough with nodules and micro-cracks. Dimensional tolerance is uncontrolled. Wear surface does not perform as expected. | Always specify "PLATE OVERSIZE, GRIND TO FINAL DIMENSION" for hard chrome. Chrome is deposited oversize intentionally. |
| Plating cast iron without extended pre-treatment | Uneven coating, blistering, poor adhesion, porosity traps solution that causes after-rust. Parts look plated but the coating flakes off. | Inform the plating shop that the material is cast iron. Request extended acid pickle and copper strike. Expect higher cost. Not all shops handle cast iron well -- verify capability. |
| Specifying decorative chrome without nickel underlayer | Chrome alone is porous and provides almost no corrosion protection. It cracks under slight deformation. Rust shows through in days on outdoor parts. | Decorative chrome is always applied over nickel (and often over copper first). The standard stack: copper strike + nickel (10–20 μm) + chrome (0.25 μm). The nickel provides the corrosion barrier. |
| Not calling out plating thickness on the drawing | Shop applies their default thickness, which may be too thin for your corrosion requirement or too thick for your tolerances. No way to reject non-conforming parts. | Always specify: "ZINC PLATE PER ASTM B633, TYPE II, SC 2 (YELLOW CHROMATE), 0.0005 IN MIN" or equivalent. Thickness, standard, and chromate type should all be on the drawing. |
| Delaying baking more than 4 hours after plating | Hydrogen diffuses deeper into the steel. Baking becomes less effective. Some hydrogen becomes permanently trapped. Embrittlement risk remains. | ASTM B850 requires baking within 4 hours of plating completion. Coordinate logistics with the plating shop. If parts must ship to another facility for baking, specify the time limit on the PO. |
| Masking cost not considered in quoting | Unit price is higher than expected because masking labor was not factored in. Margin erosion on the order. | Identify masked features during quoting. Each masked surface or hole adds $1–10. For parts with 5+ masked features, the masking cost can exceed the plating cost. |