Guide
How do you get nickel plating on aluminum to stick?
Nickel plating on aluminum is a nickel coating, electroless or electrolytic, applied over an aluminum part after the natural oxide has been removed and replaced by a thin zinc or tin immersion layer. Short answer: it sticks when the plater controls cleaning, etching and the zincate step for your exact alloy, and it fails when the drawing leaves those steps, the alloy and the adhesion test unstated.
By Saola Manufacturing engineering team · Published · Updated
Aluminum is one of the harder metals to plate well, and the reason is chemistry, not workmanship. This guide covers what happens before the nickel goes on, which aluminum alloys and part forms cause trouble, how to write the drawing note and RFQ, how to accept the parts, and when a hard anodize is the better answer. The choice between electroless and electrolytic nickel itself is covered in the electroless vs electrolytic nickel guide; this page assumes you have already picked one.
On this page
Why does nickel not stick to bare aluminum?
Aluminum grows a tight oxide film within moments of being cleaned, and nickel deposited on that film has almost no bond to the metal underneath. Every working process for nickel on aluminum is built around removing the oxide and keeping it from coming back until the first metal layer is down.
A clean aluminum surface reoxidizes in air and in rinse water, so a simple acid dip followed by a nickel bath leaves the nickel sitting on oxide. The parts may look fine and pass a casual visual check, then blister in service or peel when a thread is tapped or a press fit is pushed in. That failure mode is the reason the pretreatment matters more on aluminum than on steel or brass.
The industry answer is an immersion layer. The scope of ASTM B253, the ASTM guide to preparing aluminum alloys for electroplating, covers cleaning and conditioning treatments and the immersion or strike procedures that enhance the adhesion of metals later applied by electrodeposition or by autocatalytic reduction. In plain terms: the part is cleaned, etched, desmutted, then dipped in a zincate (zinc immersion) or stannate (tin immersion) solution that dissolves the oxide and leaves a thin zinc or tin film. The nickel then plates onto that film, not onto oxide.
What does the zincate step involve, and what is a double zincate?
A double zincate puts the zinc film down, strips it in nitric acid, and applies it again, which gives a finer and more even zinc layer than a single dip. Published adhesion work found the double treatment improved adhesion on most alloys tested, so it is the default many platers use for engineering parts.
A 2010 study in Materials Transactions (Murakami and co-authors) zincated four wrought aluminum alloys and peel-tested the electroless nickel-phosphorus coating on top. Double zincate improved adhesion, except on the 7075-type alloy, and the authors concluded that a uniform oxide film before zincating gives a more uniform zinc film. The practical lesson is that the pretreatment is alloy-specific: the etch, the activation acid and the zincate chemistry that work on one alloy may underperform on another.
The table below sums up how each step can fail and what the buyer can ask for. It is a planning list, not a process specification; the plater writes the actual process sheet.
| Step | What it does | Typical failure sign | What the buyer can ask for |
|---|---|---|---|
| Soak clean and degrease | Removes machining coolant and oils | Patchy coverage, stains near pockets | Coolant type named, parts cleaned before shipping to the plater |
| Etch and desmut | Removes oxide and alloy smut left by the etch | Dark smut, rough or pitted surface | Etch suited to the alloy; no etch on precision bores without agreement |
| Zincate (single or double) | Replaces oxide with a thin zinc film | Blisters, flaking at edges | Double zincate, or the plater's stated pretreatment for your alloy |
| Nickel strike or coating | First nickel layer bonded to the zinc film | Peeling under bend or tap | Adhesion test method stated on the PO |
| Post-plate bake, if specified | Improves adhesion or hardness | Blisters appear after baking | Bake class stated, and limited to what the alloy temper allows |
Which aluminum alloys and part forms are hard to plate?
High-silicon castings, porous die castings and some 7000-series alloys are the usual problem cases. Because the right pretreatment depends on the alloy, the drawing must name the exact alloy and temper, not just "aluminum".
Castings bring two problems. Silicon particles do not take the zincate the same way aluminum does, and porosity traps cleaning and plating solutions that later weep out and blister the coating. The Aluminum Anodizers Council FAQ makes the same point for anodizing: castings are often porous, and alloy 380 at about 8% silicon anodizes with gray mottled areas. Die cast ADC12 and A380 housings that are machined after casting expose fresh subsurface porosity on every machined face, so a cast-and-machined part can plate worse than the as-cast skin.
Geometry matters as well. Blind holes and cross-drilled passages hold rinse water and zincate, and tapped holes can trap solution in the thread roots. If a feature does not need nickel, mask it or plug it rather than hoping the plater rinses it out. Sharp internal corners and deep slots also see less solution exchange, which is one more reason to state the significant surfaces on the drawing.
How do you write the nickel callout for an aluminum part?
Name the nickel standard and its type, class or thickness, then add the base alloy and temper, the pretreatment expectation, the adhesion test and the significant surfaces. A callout that only says "nickel plate" leaves the most important decisions to the plater.
A complete note for an aluminum part has five parts. First, the coating standard and grade, for example electroless nickel per ASTM B733 with its service condition and phosphorus type, or electrodeposited nickel per ASTM B689 with a minimum thickness. Second, the base metal and temper, written on the drawing, because the plater picks the etch and zincate from it. Third, a pretreatment note, such as "prepare per ASTM B253, double zincate or equivalent immersion process", which tells the plater you know the step exists without writing their process for them. Fourth, the adhesion test. Fifth, the significant surfaces and the masked features.
Watch the post-plating bake. ASTM B733 assigns heat-treatment classes for adhesion and hardness improvements, but a hot bake can over-age a heat-treated alloy such as 6061-T6. If you call a bake class, check it against the alloy temper you need in the finished part, and ask the plater to confirm it in writing.
Dimensions need a note too. Say whether the drawing dimensions apply before or after plating, and leave room in the tolerance for the coating on bores and threads. The masking guide covers how to mark threads and bores that must stay bare.
How do you inspect and accept nickel on aluminum?
Ask for an adhesion test from each lot, coating thickness readings at named locations, and a visual check for blisters after any bake. Adhesion is the property that fails on aluminum, so it is the one to test every time.
The scope of ASTM B571 covers simple, qualitative tests for the adhesion of metallic coatings, such as bend, heat-quench and file tests; it suggests choosing the test by how the part will be used, for example a bend test for a part that is formed later or a heat-quench test for a part that will be heated. Because these tests can damage the part, agree whether they are run on sacrificial parts or on test coupons of the same alloy processed in the same load. Coupons are cheaper, but a coupon of wrought sheet tells you little about a die cast housing, so for castings ask for a sacrificial part.
Thickness can be checked with an instrument the plater has qualified for that nickel type on an aluminum base, or by cross-section on a sacrificial part. Name the measurement points on the drawing, because electrolytic nickel on a long part is not the same thickness at the ends and the middle. On receiving inspection, look for blisters, dark spots and edge lifting, and tap a few threads: a flake in a thread root is the cheapest early warning you will get.
When should you anodize aluminum instead of nickel plating it?
Anodize when you need wear and corrosion resistance on aluminum and can live with a nonconductive, slightly grown surface. Choose nickel when you need electrical conductivity, solderability, a metallic look, or a base for a later layer.
Anodizing grows the oxide out of the part itself, so there is no zincate step and no adhesion boundary to fail in the same way. The Aluminum Anodizers Council notes that the oxide is thicker than the aluminum consumed, so dimensions change, and that hardcoat has roughly the hardness of nitrided steel. The trade-offs run the other way for nickel: it conducts, it can be soldered or plated over, and electroless nickel coats bores evenly where the solution circulates.
| Requirement | Anodize (Type II or hardcoat) | Nickel over zincate |
|---|---|---|
| Electrical contact or grounding | Insulating oxide; mask contact areas | Conductive |
| Wear on a sliding face | Hardcoat is the usual choice | Electroless nickel, often heat treated |
| Soldering or a later plated layer | Not suitable | Suitable |
| Die cast, high-silicon alloy | Mottled, uneven film | Possible, but adhesion risk is higher |
| Adhesion failure mode | Integral oxide, no plated interface | Depends on zincate quality |
| Color options | Dyed colors available | Metallic only |
How does Saola handle nickel plating on aluminum parts?
Finishing at Saola is a managed process done at partner plants against a fixed menu, and that menu names electroplating and anodizing on aluminum but not electroless nickel. Raise nickel on aluminum at drawing review so the zincate route, alloy and adhesion test are agreed before the order.
The surface finishing page lists the eight finishes the partner plants apply: electroplating, gold plating, zinc plating, RoHS blue trivalent chromate, oil-dip rust protection, anodizing on aluminum, powder coating and pad printing. Electroless nickel is not on that menu, so a drawing that depends on it needs a direct answer at drawing review. Whether electrolytic nickel on your aluminum alloy can be run under the electroplating line is also confirmed at drawing review; name the nickel type, the alloy and the adhesion test you expect in the RFQ. The machining itself is done at the lead CNC plant: the CNC milling page explains that most milling work there is second-operation machining on castings, which is exactly where porosity under a plated face shows up. Send the drawing through the quote form.
Frequently asked
Can you nickel plate a die cast aluminum housing?
Often, yes, but it carries more risk than wrought bar. Porosity opened by machining traps solution and can blister the coating, so ask for a sacrificial-part adhesion test and consider impregnation sealing before plating.
Is a stannate (tin immersion) process better than zincate?
It is an alternative named alongside zinc immersion in the ASTM B253 guide. Which one gives better adhesion on your alloy is a plater's call; specify the result you need and the adhesion test, and let the plater state the route.
Does electroless nickel on aluminum need a bake?
Not always. A bake can improve adhesion and hardness, but it can also over-age a heat-treated alloy, so state the bake class only after checking it against the temper you need.
Why did my nickel-plated aluminum parts blister weeks after delivery?
The usual causes are trapped solution in pores or blind holes, an oxide layer left under the coating, or galvanic attack through a pinhole in a wet environment. A cross-section of a failed part usually shows which.
Can nickel go straight on top of an anodized surface?
No. The anodic film is an insulating oxide, which is exactly what the zincate step exists to remove. Strip the anodize first, or mask the areas that need nickel before anodizing.
Sources
- ASTM B253-11, Standard Guide for Preparation of Aluminum Alloys for Electroplating (scope), ASTM International, 2011 (reapproved 2022), accessed November 2026
- ASTM B571-23, Standard Practice for Qualitative Adhesion Testing of Metallic Coatings (scope), ASTM International, 2023
- ASTM B733-22, Autocatalytic (Electroless) Nickel-Phosphorus Coatings on Metal (abstract), ASTM International, 2022
- Murakami, Hino, Furukawa, Kanadani, Effects of Alloying Elements in Aluminum Alloys and Activations on Zincate Treatment and Electroless Nickel-Phosphorus Plating, Materials Transactions 51(1), 2010
- Anodized Aluminum FAQ, Aluminum Anodizers Council, accessed November 2026