Testing adhesion of electroless nickel plating

A. Hi Gary. I think it's simply a matter of SAE recognizing that it's possible to electroless nickel plate strips that are so extremely thin that 4X their thickness would still be next to nothing. So rather than requiring that the plating withstand essentially a tight hammered crease, they don't require bending over a mandrel smaller than 0.4"

Read on, and enjoy that the finishing world is a richer place for once having had Ken Vlach answering questions here :-)

Luck & Regards,

A. Hi Jake,
Bend testing is described in ASTM B733 (para. 9.4.1) & ASTM B571 (para. 3.1) and MIL-C-26074 [on DLA]E (paragraphs 4.5.2 & 3.3). A summary:

>From MIL-C-26074E:
"4.3.2 Separate specimens. When the coated parts are of such form as to be not readily adaptable to a test...The separate specimens shall be of a basis material equivalent (see 6.6) to that of the parts represented....The separate specimens shall be strips approximately l-inch wide, 4-inches long, and 0.04-inch thick...These specimens shall be introduced into a lot at regular intervals prior to the cleaning operations preliminary to coating and shall not be separated therefrom until after completion of the processing."
[Note ASTM B733 size: approx. 25-50 mm wide, 200-300 mm long, and 3-6 mm thick]

"4.5.2 Adhesion test. The test specimen shall be bent 180° over a mandrel diameter 4 times the thickness (0.4 inch minimum) of the specimen."

Adhesion is then evaluated per MIL-C-26074E para. 3.3 or the referenced ASTM paragraphs.

For this purpose, a mandrel is a round bar or pipe (typically, clamped in a vise), 0.4" OD for thin strips up to 0.1" thick or 0.5" OD for 0.125" thick strip. To my knowledge, nobody uses strip thicker than 0.125" or 3 mm.

Laboratories doing salt spray & thickness qualification testing can furnish test panels -- the same size can be used for bending & thickness tests, or a 4x6" panel (salt spray test size) can be cut into 4 bend test panels. The panel material(s) must ~match the production material(s) in chemical composition and surface finish prior to coating application, MIL-C-26074E, para. 4.3.2 & 6.6, and ASTM B733, para. 8.3.