ISO 8994:2011 pdf download – Anodizing of aluminium and its alloys – Rating system for the evaluation of pitting corrosion一Grid method.
ISO 8994 specifies a grid rating system that provides a means of defining levels a
performance of anodic oxidation coatings on aluminium and its alloys that have been subjected to corrosion
This rating system is applicable to piting corrosion resulting from
accelerated tests,
-exposure to corrosive environments, and
practical service tests.
This Intermational Standard takes into account only ptting corrosion of the basis metal resulting from
penetration of the protective anodic oxlidation coating.
ISO 89931 describes a similar rating system based on defined chart scales.
NOTE2 The gid rating system is frequenty used for rating the rsulis of short-term corrosion tests for relatvely thin
anodic oxidation coating, such as those used in the automotive industry.
2 Terms and definitions
For the purposes of this document, the fllwing terms and definitions apply.
significant surface
part of the article covered or to be covered by the coating, for which the coating is essential for serviceability
and/or appearance
NOTE 1 Adapled from ISO 2064:1996/1. definition 3.1.
NOTE2 The edges of an article are not normally included in the signficant surface.
corrosion pit
surface corrosion defect at which the anodic oxidation coating is penetrated
Discoloration or other surface delects which do not penetrate the anodic coating do not count as corrosion pits.
3 Procedure for rating
3.1 Sample preparation
A sample area of more than 5 000 mm2 is required.
Use one of the fllowing methods to remove corrosion products or deposits on the surface so that corrosion
pits may be clearly discermed: :
a) wipe with a slurry of fine pumice to abrade away corrosion products and dirt, then rinse in clean water and
air dry;
b) dip for 5min to 10 min in 30 % nitric acid, prepared by diluting 1 volume of concentrated nitric acid
(P20= 1.40 g/m) with 1 volume of water at 20 °C to 25 °C; rinse and dry as indicated in a);
c) dissolve the anodlic oxldation coating in a hot phosphoric acid/chromic acld mixture; rinse and dry as
indicated in a) so that ptting in the aluminium substrate may be discerned;
ISO 210621 describes the preparation and use of this reagent for the purposes of dissolution of the
anodic oxidation coating.
This method is particularly useful for dark-coloured anodic oxidation coatings.
WARNING一Chromium(VI) is toxic and shall be handled properly. Chromium(VI) solutions are
hazardous to the environment and severely hazardous to waters.
d) wipe with soft textile gauze dipped in dilute hydrochloric acid solution (100 ml of 35 % to 37 % HCI, made
up to 1 000 ml with disilled water or deionized water) to remove deposited copper, then rinse and dry as indicated in a).
3.2 Determination of grid rating
Place a preprinted transparent grid with an area of at least 5000 mm2, and with grid squares of 5 mm x 5 mm, over a selected area of the significant surface of the prepared test specimen. Count the number of grid squares occupied by one or more pits. disregarding effects on the edges of samples. Calculate the percentage of defective squares, ie. grid squares containing pits, using Formula (1):
4 Expression of results
Express the result of the examination as the grid rating and/or the percentage of defective squares, as appropriate.
5 Test report
The test report shall include at least the fllowvin information:
a) a reference to this International Standard;
b) the type and identification of the product tested and, where appropriate, the anodizing. exposure and corrosion test procedure;
c) the method of sample preparation used (see 3.1);
d) the grid rating and/or the percentage of defective squares (see Clause 4);
NOTE The acceptable grid rating will normally be specified in the relevant corrosion test or product specification.
e) the date of the test.