ISO 8994:2018

Overview

ISO 8994:2018 specifies a grid rating system to assess pitting corrosion of anodic oxidation coatings on aluminium and its alloys. The method quantifies the occurrence of corrosion pits that penetrate the protective coating after exposure to accelerated tests, corrosive environments or practical service conditions. The standard is widely used for short-term corrosion tests and thin anodic coatings (for example in the automotive industry).

Key Topics

• Scope and focus: only pitting corrosion of the base metal caused by penetration of the anodic oxidation coating is considered.
• Specimen preparation: test areas greater than 5 000 mm² are required. Surface treatments to reveal pits include:
  • wiping with a fine pumice slurry then rinsing and drying;
  • dipping in diluted nitric acid (specified concentration and rinse procedure) followed by pumice cleaning;
  • dissolving the anodic coating in a hot phosphoric/chromic acid mixture (useful for dark coatings) and subsequent cleaning (note hazard of Chromium(VI));
  • wiping with soft gauze dipped in dilute hydrochloric acid to remove deposited copper, then rinsing and drying.
• Grid application and counting: a transparent grid (5 mm × 5 mm squares) is placed over a selected area (minimum 5 000 mm²). Count the number of grid squares that contain one or more corrosion pits (defective squares).
• Calculation: percentage of defective squares D = (Ns / Nt) × 100, where Ns is number of defective squares and Nt is total squares.
• Grid rating conversion: the percentage (or number of defective squares out of a reference 200-square area) converts to a grid rating number using the table in ISO 8994:2018 (rating numbers include 0, 1, 2, 3, 6, 12, 25, 50, 100, 200 corresponding to increasing defect levels).

Applications

• Assessing comparative performance of anodic oxidation coatings after salt spray, humidity or other accelerated corrosion tests.
• Routine quality checks for thin anodized layers in automotive and consumer products where short-term tests are common.
• Producing consistent, reproducible results for procurement specifications and conformity assessment by reporting percentage defective squares, number of defective squares and/or the grid rating number.

Related Standards

• ISO 8993 - chart method for pitting corrosion rating (alternative approach).
• ISO 2106 - reagent preparation and gravimetric determination related to anodic coating analysis.

Bold reporting and consistent test documentation are essential. Test reports should reference ISO 8994:2018 and record specimen identification, preparation method, calculated percentage of defective squares, grid rating number (if used), unusual observations and test date.