Case Histories » Automotive

Filiform Corrosion of Coating on Aluminum Structure

Corrosion initiated at damaged area of rivet
Semi-Circular shaped corrosion sites had initiated at damaged area of the coating at the outer edge of the rivet
Scanning electron microscope view of corroded aluminum
View of the Cavity beneath the 2-coat system indicating significant corrosion of the aluminum (scanning electron microscope)
Magnified view of micro-cracking of coating with filiform tunnels
Magnified view of Corrosion Site . Micro-cracking of the coating along with filiform tunnels (denoted by arrow)

Material

3004 aluminum alloy in the H291 temper.

Environment

Exterior of vehicle driven on the prairies.

Background

Since Testlabs International had analyzed coating failures of the one-coat system used in the past, the company implemented a two-coat paint system for the aluminum sheets.

Service Life

Ranged from five to eight months.

Findings

The two-coat paint system, which specifies a total dry film thickness of 1.1- 1.3 mils, has reduced the number of corrosion sites as compared to the one-coat paint system.

With the one-coat paint system, there were 100+ corrosion sites / ft2

With the two-coat paint system, there are five to 17 corrosion sites/ ft2