Peel-off strength of Electroless Nickel on Aluminum

A. I can exert more pressure than 1-2 PSI with my fingernail. I doubt that figure (as stated) greatly. Poor adhesion on aluminum is a result of either the cleaning or the zincate step. There is no good visual indication of the quality of the zincate, so it is done poorly in a number of shops. We used an alkaline EN strike followed by a mid phosphorous EN and had excellent adhesion.

A. Shooting this plater might be an appropriate corrective action. More profitably, accept payment for not publishing their name with their inane 1-2 PSI comment on the Internet. Did they certify to some standard?

To withstand plating stresses (coating internal stresses and CTE stress cooling from plating temperature), EN-to-aluminum adhesion should be 5000-10,000 psi. Adhesion can be improved and internal stress relieved by baking (e.g., 120 °C for 1 hour for heat-treatable Al alloys), but this will reduce solderability. The 180° bend test and uses such as die/mold and engine piston coating suggest adhesive strengths of 25,000-50,000 psi are achievable.

Many possible causes of adhesion problems; see letters 878, 4074, 4487, 12880, 14954, 21801, 22414, 23303, 27627...

For an EN-plated Al block, the quench test can be used: Heat the item for 1 hour at 250 °C, then "quench in room temperature water. The appearance of blisters or peeling is evidence of inadequate adhesion." -- Electroless Plating, pp. 180-181, Mallory & Hajdu, eds. (1990).

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
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A. The shear bond strength of properly pretreated aluminum is at least 16,000 psi. Proper pretreatment of aluminum requires cleaning, deoxidation, etching and a double zincate immersion to guarantee good adhesion.

A. I believe the current European spec. is ISO 4527:2003
( BS EN ISO 4527:2003 in the UK) Metallic coatings. Autocatalytic (electroless) nickel-phosphorus alloy coatings. Specification and test methods.

Description: Metal coatings, Coatings, Electroless plating, Nickel, Nickel alloys, Phosphorus, Heat treatment, Surface defects, Chemical composition, Chemical analysis and testing, Calibration, Density, Physical properties of materials, Adhesion tests.
Cross references to ASTM, EN and ISO specs.

This International Standard does not apply to autocatalytic nickel-boron alloy coatings, nickel-phosphorus composites and ternary alloys.

UPDATE Through some testing and help from a local university we have discovered that our failures are actually due to shock fracture not peel off.

The failure is of the nickel/phosphorus coating which is made shock sensitive due to the interaction of phosphorus and aluminium during the soldering process and stressed by the relative thermal expansions of the aluminium and nickel. This stress is 'locked in' by the freezing of the solder attaching it to the circuit board and a total failure is then initiated by a fairly small shock to the assembly.

A. If the EN let go from the aluminum, I would question the quality of the zincate step. I am a very strong believer in the use of the alkaline EN strikes. It is excellent for improving adhesion. The ammonia [on eBay or Amazon] smell is strong though.

A. Good adhesion is probably required, too.

If selecting an electroless nickel plater, ask to see their monthly lab test reports per MIL-C-26074; most EN platers have the thickness, adhesion and corrosion tests run on both steel & aluminum test coupons.

Specify 0.0010-0.0012" mid-phosphorous EN per Class 4 of either ASTM B733 or MIL-C-26074 [on DLA]. Class 4 require an adhesion bake suitable for heat treated aluminum, e.g., 120 °C (250 °F) for 1.5 hours. Have a scrap part or 6061-T6 test coupon included in the processing and adhesion tested (somewhat destructive) periodically. If adhesion is a problem, search this site for 158 Letters on 'adhesion electroless nickel aluminum'.

Any heat distortion is actually the release of residual stresses from the initial quenching which have been unbalanced by machining. If this occurs, add a mild stress relief bake slightly warmer than the final adhesion bake, e.g., 125 °C (260 °F) for 2 hours, prior to final machining.