Client case: Corrosion analyses of powder coated aluminium

Image 01Product: Exterior sills and vertical wall panels
Material: Powder coated aluminium plate
Sector: Construction, real estate
Where? Apartment building, North Sea coastal area
First observations: Grey-white staining and local detachment of the coating
When observed? finishing phase of the building, prior to delivery to the owner
Analysis procedure: Visual and stereomicroscopic analysis, light optical microscopy with classic lighting as well as in darkfield-mode, scanning electron microscopy with spectral analysis (SEM-EDS)

Main conclusions

Corrosion was due to the presence of large gas cavities and an inhomogeneous (size) distribution of barium sulphate crystals in the coatings. 

Due to the varying outdoor temperature the gas cavities will expand and shrink, which subjects the coating to varying stresses. These stress variations, combined with the limited amount of coating material at the cavity positions has led to coating cracks at/near some of the larger cavities. 

It was also observed that several barium sulphate crystals - which are also added in function of gloss optimisation - were quite large compared to the coating thickness. Barium sulphate crystals with a size of one fourth of the coating thickness were regularly observed. At the same time an inhomogeneous distribution of the barium suplhate crystals was observed. At several postions some of the larger barium sulphate crystals were aligned, together forming an almost continuous line from the aluminium substrate to the coating surface, which led to the creation of ideal propagation paths for cracking of the coating. 

The local corrosive attack of the sills and wall panels was subsequently caused by the penetration of marine humidity/moisture through the coating cracks up to the aluminium substrate.

Recommendations for repair

Given the nature of the coating damage described above and as the aluminium was already severely attacked at several positions, it was advised to replace the complete sills and wall panels. Local repair of damaged areas would not have given the guarantee that the remaining original coating areas would not be affected by the observed failure phenomena.

Picture at the top of the blog: Stereomicroscopic enlargement of a failure area

Picture top left: Light optical microscopy, darkfield image of a section zone with cracking of the coating (large white area = corrosion products of aluminium)
Picture top right: Light optical microscopy, darkfield image of a section zone with gas cavities in the coating
Picture bottom left: SEM-BS-image of a section, with the coating on top and corrosion products (dark grey) and aluminium (light grey) below
Picture bottom right: EDS-mapping of barium for the section zone of the SEM-BS-picture / The EDS-mapping for sulphur is similar  








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