Independent Corrosion Solutions

Consultancy in all aspects of Corrosion Investigations

Case Histories

Below are just two of the recent case histories we have worked on. This lists the problem and the solution.

Case History 1: Deterioration of Anodized Aluminium

The Problem
We were approached by the Client (a cleaning firm, contracted to maintain anodized aluminium panels). They found a problem in cleaning and restoring to good condition (cosmetic appearance) panels of corrugated anodized aluminium for external curtain walling. The sites were located throughout the UK such that a wide range of atmospheric conditions were experienced by the panels.

Various proprietary cleaners had been used with varying degrees of success – the question remained for how long would the appearance be maintained and would there be long-term damage to the anodized layer?

The Solution
Firstly, a literature review was undertaken, with special reference to all relevant British Standards. Subsequently, the chemical composition of the substrate and coating thickness were then determined.

Quantitative analysis of the as-received bulk aluminium substrate revealed that it contained 99% aluminium (i.e. it did not conform to the 99.5% called for in the Tender) which corresponds to “commercially pure aluminium” (designated 1C) which would give ‘very good’, but not ‘excellent’ results for protective anodizing.
Subsequently, high voltage flaw technique, optical microscopy and electron microscopy were used to characterize the anodized material. Pristine (Figure 1), after exposure to the atmosphere (Figure 2) and exposed to the atmosphere, subsequently cleaned with recommended proprietary cleaner Figure 3).

Alloy surface 1

Figure 1: Electron micrograph of surface of pristine material.
Note pores(x 1600)

Alloy surface 2

Figure 2: Optical micrograph of cross-section of sample (x 900), showing oxide thickness and porosity in oxide. Also note defects in underlying metal. Oxide thickness as measured by electron microscope, optical microscopy and Elcometer = 6μ , 9.10 μ and 5.9 μ respectively.

Alloy surface 3

Figure 3: Electron micrograph of front side of sample, previously exposed to atmospheric corrosion, subsequently cleaned with proprietary cleaner, showing numerous pores (x 1300)

Conclusions
It was found that the thickness of the anodized layer was below that of the recommended 25 μ as specified in BS 3987:1991. Defects or ‘holidays’ were found in the anodized layer of all specimens examined (including pristine material) extending right from the outside surface of the anodic coating to the underlying aluminium alloy. Since the as-received material was defective, it was unlikely that maintenance procedures had damaged the panels.

It is likely that tarnishing was caused by atmospheric corrosion enhanced by dirt trapped in the porous anodic oxide coating, some areas of which had been completely lost.


Case History 2: Examination of failed mild steel radiators

The Problem
Approximately 200 radiators were installed in commercial premises in Birmingham some 7 – 10 years previously, of which 15 had failed at the time, apparently by pinhole corrosion. Some 95% of the failures occurred at the lower parts of the tubular sections. No hydrogen gas was vented from the system. Biological problems were absent from the system.

Water treatment providers claimed that a nitrite/molybdate inhibitor had been added to a 25 L tank, the cover to which was discovered to have been left permanently off for some 2 – 6 months.

The Solution
Water samples (ex-system), neat inhibitor, metal samples from a ‘non-failed’ area, an actual ‘pinholed’ area (for optical microscopy) and corrosion product from the failed radiator were supplied.

Rad 1
Figure 1: Microscopic view of the "good" area showing only one-third of thickness welded
Rad 2
Figure 2: Failed specimen - far too wide for crevice, so not crevice corrosion

Conclusions
At the corrosion-site, the wide gap was not due to corrosion, but to the two welded metal butts being too far apart. The weld was too shallow and was ground off, leaving a paper-thin seal.

At the non-corroded site, the gap between the two welded metal butts is much smaller than at the corrosion site, but no crevice corrosion had occurred. Lack of weld penetration was the cause of pinhole failure.