Corrosion and Coating

Why Do Materials Need to Be Galvanized?

Zinc coating is a corrosion barrier, not a cosmetic step. This page explains why materials need to be galvanized, which machined parts belong in a plating line, and the defects that show up when the substrate or the process is wrong. Written for design engineers and buyers who have to approve a finish before the drawing is released.

Get Instant Quote Talk to an Engineer

Zinc 5–25 μmSteel and iron partsRack or barrelPost-machining step

Why do materials need to be galvanized on CNC machined engine parts

On this page

1
Galvanizing Defects: Symptom, Cause, Action
2
Why Materials Need to Be Galvanized: The Zinc Barrier
3
Hot-Dip, Electroplating, or Zinc Flake: Picking the Right One
4
Which Machined Materials Actually Take Zinc
5
Design Rules Before the Part Reaches the Plating Line
6
Step by Step: Getting a Machined Part Galvanized

Symptom check

Galvanizing Defects: Symptom, Cause, Action

Read the left column first. Most plating complaints trace back to the substrate or the racking, not the zinc bath.

Symptom	Likely cause	What to do
White rust in storage	Zinc surface exposed to moisture	Chromate passivation, dry storage, VCI wrap
Blister under the coating	Trapped hydrogen or oil in pores	Bake 190–220 °C, improve pre-clean
Patchy gray finish	Residual cutting fluid on the part	Alkaline clean, rinse, acid pickle
Thin coating on sharp edges	Low current density at corners	Rack on a jig, rotate part, add thief
Coating flakes at bend	Brittle intermetallic layer	Keep coating 5–25 μm, control bath temp
Threads no longer gauge	Zinc buildup on pitch diameter	Cut thread oversize, mask or plug
Rust at weld line	Porosity and flux residue	Grind, re-clean, spot repair after weld

Mechanism

Why Materials Need to Be Galvanized: The Zinc Barrier

Steel rusts because iron gives up electrons to oxygen and water. Zinc gives them up more easily. When the two sit in contact with an electrolyte, the zinc becomes the anode and the steel becomes the cathode. The zinc corrodes first and the steel stays intact. That is the whole idea behind sacrificial protection, and it is why materials need to be galvanized rather than simply painted.

A paint film blocks moisture only while it is unbroken. One scratch and rust starts at the exposed metal. A zinc coating keeps working at a scratch because the surrounding zinc still protects the exposed steel a few millimeters away. This self-healing behavior is the main reason galvanizing holds up on outdoor brackets, frames, and welded assemblies.

There is also a barrier effect. The zinc layer itself is dense and blocks the atmosphere from reaching the iron. Hot-dip galvanizing forms zinc-iron intermetallic layers that bond metallurgically to the steel, so the coating does not peel like a loose paint film.

The trade-off is dimensional. Any coating adds thickness. On a part held to ±0.005 mm, a 25 μm zinc layer on each side eats 50 μm of your tolerance. That is why the decision belongs on the drawing, not in the finishing shop.

1
Sacrificial protectionZinc corrodes before steel, even at scratches
2
Barrier protectionDense zinc layer blocks moisture and oxygen
3
BondingHot-dip forms zinc-iron layers, not a paint film

Process choice

Hot-Dip, Electroplating, or Zinc Flake: Picking the Right One

Hot-dip galvanizing dips the finished part in molten zinc around 445–460 °C. Coating thickness typically lands between 45 and 85 μm, which is heavy protection for structural steel. It suits weldments, base plates, and handrail sections where a few tenths of a millimeter do not matter. It does not suit tight-tolerance machined surfaces, because the heat can move them.

Electroplated zinc runs in a bath near room temperature and deposits 5–25 μm. It holds far better dimensional control, so it fits CNC machined parts, fasteners, and small housings. Barrel plating handles thousands of small parts at once. Rack plating gives more uniform coverage on larger or oddly shaped parts and lets you control where the coating goes.

Zinc flake coatings are applied as a dip-spin then cured. They give 8–15 μm with no hydrogen embrittlement risk and no rack marks, which makes them common on high-strength fasteners and automotive brackets. If your part sees salt spray testing above 720 hours, this family is worth a look.

The choice is rarely about corrosion alone. It is about how much thickness the assembly can absorb, whether the part is heat-sensitive, and how many pieces you are running. A 10,000-piece run of small pins and a single 4,000 mm frame do not go through the same line.

1
Hot-dip45–85 μm, structural steel, heat is not an issue
2
Electroplated zinc5–25 μm, tight tolerances, rack or barrel
3
Zinc flake8–15 μm, high-strength fasteners, no embrittlement

Substrate

Which Machined Materials Actually Take Zinc

Carbon and low-alloy steels are the natural home for galvanizing. Grades like 1018, 1045, 4130, 4140, and 4340 all accept zinc well once the surface is clean. The higher the alloy content, the more careful the pickle step has to be, because aggressive acid can attack grain boundaries.

Stainless steel is a different story. Grades 303, 304, and 316 already form a passive chromium oxide layer. Zinc does not bond to it reliably, and the plating usually looks patchy. If you need stainless for corrosion resistance, you generally do not need galvanizing. If you need a conductive or decorative layer, electroless nickel or passivation is the better fit.

Aluminum and its alloys, including 6061, 7075, and ADC12, cannot be galvanized in the ordinary sense. The oxide layer forms instantly and blocks adhesion. Aluminum gets anodized instead, which builds an oxide layer on purpose. Copper, brass C36000, and titanium follow the same logic: pick the finish that matches the metal, not the one you used last time.

Magnesium alloys such as AZ31B and AZ91D are the worst case. They are more active than zinc, so zinc offers no sacrificial benefit and may accelerate attack. Those parts need a dedicated conversion coating.

1
Good fit1018, 1045, 4130, 4140, 4340 carbon and alloy steel
2
Poor fit303, 304, 316 stainless; aluminum; copper; titanium
3
Wrong fitMagnesium AZ31B and AZ91D, more active than zinc

Design rules

Design Rules Before the Part Reaches the Plating Line

Zinc builds up on edges and thins in recesses. Current density is highest at corners, so a sharp 90° edge can carry two to three times the coating of a flat face. Break edges to a 0.5 mm chamfer or radius and the coating evens out. Blind holes deeper than twice their diameter will not plate at the bottom; specify a through hole or accept bare metal there.

Threads are the most common complaint. A zinc layer adds roughly 4 to 8 μm per flank on an electroplated part, which changes the pitch diameter and can stop a nut from running on. Either cut the thread with extra allowance, or mask and plug it. For hot-dip, threads almost always need to be cut oversize before dipping.

Trapped hydrogen is a real risk on parts above 32 HRC or roughly 1,000 MPa tensile strength. Acid cleaning puts hydrogen into the steel, and plated parts can crack under load days later. Bake at 190–220 °C within four hours of plating, typically for four hours or more, before the part sees service.

Finally, mark the datum and any ground surface as no-plate. Once you have a 15 μm layer on a bearing seat, the fit is gone. Masking tape, plugs, and a clear note on the drawing cost nothing compared to re-machining a finished part.

1
Chamfer edges0.5 mm radius keeps coating uniform at corners
2
Plan threadsCut oversize or mask; zinc changes pitch diameter
3
Bake hard parts190–220 °C within four hours if above 32 HRC
4
Mask fitsBearing seats and datums should stay bare

Workflow

Step by Step: Getting a Machined Part Galvanized

The sequence matters more than any single parameter. Skipping the clean or the bake is how most failures start.

1
1. Confirm the substrate gradeCheck the material certificate against the drawing. 1018, 1045, 4130, 4140 and 4340 are all fine. If the print says 304 or 6061, stop and switch the finish before any machining starts.
2
2. Decide the coating thicknessElectroplated zinc at 5–25 μm for tight parts, hot-dip at 45–85 μm for structural work. Write the range on the drawing, not the word 'galvanized' alone.
3
3. Set the machining allowanceLeave 0.05–0.10 mm on surfaces that will be plated if the final dimension matters. On threads, cut to the pre-plate pitch diameter so the nut still gauges after coating.
4
4. Machine and deburrBreak all edges to 0.5 mm. Remove burrs from cross holes and slots. A burr that falls off in the bath leaves a bare spot under the coating.
5
5. Clean before shippingRemove cutting fluid with an alkaline wash. Oil left in a blind hole will outgas in the bath and cause a blister that no amount of zinc can fix.
6
6. Bake if the part is hardAbove 32 HRC, bake at 190–220 °C within four hours of plating, four hours minimum. Log the time between plating and bake.
7
7. Inspect and recordCheck coating thickness on a flat face and an edge. Verify threads with a go/no-go gauge. Ask for salt spray data if the part is going outdoors.
8
8. Protect in transitWrap in VCI paper and keep the parts dry. Zinc that sits wet in a cardboard box will show white rust before it reaches the assembly line.

FAQs

Galvanizing Questions Engineers Ask

Does galvanizing change the dimensions of a machined part?

Yes, by the coating thickness on every exposed surface. Electroplated zinc adds roughly 5–25 μm per side, so a 20 mm shaft can grow 10–50 μm on diameter. Hot-dip adds far more, often 45–85 μm per side.

If a feature is held to ±0.005 mm, mask it or machine it undersize before plating. Threads, bearing seats, and dowel holes are the three places this bites hardest.

Can I galvanize stainless steel or aluminum?

Not in the usual sense. Stainless steel already carries a passive chromium oxide layer, so zinc does not bond reliably and the result is patchy. Aluminum forms its oxide instantly, which is why anodizing is used instead.

If you need corrosion resistance on 304 or 6061, the right answer is passivation for stainless and anodizing for aluminum. Zinc belongs on carbon and low-alloy steel.

What causes white rust on galvanized parts?

White rust is zinc oxide and zinc hydroxide forming on the surface. It needs moisture and limited airflow, which is exactly what happens when wet parts are stacked in a closed box or a humid warehouse.

The fix is not more zinc. Passivate the surface, dry the parts fully after plating, and wrap them in VCI paper. Keep the pallet off a concrete floor in humid weather.

Why did my plated part blister after a few days?

Blistering usually means something was trapped under the coating: oil, cutting fluid, or hydrogen. A blind hole with residual coolant will outgas in the plating bath and lift the zinc as it cures.

Improve the pre-clean, add an alkaline soak and a proper rinse, and check that blind holes drain. If the steel is above 32 HRC, confirm the post-plate bake happened within four hours.

Do I need to bake parts after zinc plating?

Only if the steel is high strength. The threshold most shops use is around 32 HRC or 1,000 MPa tensile. Below that, hydrogen is not a practical concern.

Above it, bake at 190–220 °C within four hours of plating, usually for four hours or more. The clock starts when the part leaves the bath, not when it reaches the oven.

How long does galvanized steel last outdoors?

It depends on the environment and the coating thickness, not on a single number. A 5 μm electroplated layer in a coastal atmosphere will not last as long as an 85 μm hot-dip layer in a dry inland site.

For outdoor structural work, specify hot-dip and ask the plater for salt spray data on the actual process. For indoor parts, a thinner electroplated layer is usually enough.