Glycol-Based Flux Paste Interaction with Aluminium at High-Humidity Storage Conditions
Leszek Orman, PhD, Solvay Fluor GmbH, Germany
Introduction
It has been reported on several occasions that when a flux paste made with glycols mixture as liquid carrier is applied on aluminium and afterwards the parts are kept in storage for a prolonged time, a greyish discoloration appears in the surface areas where the paste was applied. This discoloration when developed to a higher degree can lead to serious problems in subsequent brazing. Also it was observed that higher humidity and higher temperature increases the degree of that discoloration.
Industrial example
Fig. 1: Folded B-type tubes fluxed during folding with a glycol based paste and stored forabout three weeks
The greyish discoloration evident after opening the tubes was analysed at Scanning Electron Microscope [SEM].
Fig.2: Secondary Electron Image of greyish discoloration and EDX diagram of an area(marked with a red square)
It can be observed that apart from flux elements (Al, K, F) there is a strong peak fromoxygen. An EDX diagram performed at the same parameters but on the surface next tothe greyish discoloration just revealed only presence of aluminium. This observationallows for conclusion that there must be some oxidation process in areas coated with fluxpaste.
Laboratory investigation
Experiments were performer with: small coupons (25mm x 25mm x 0.4mm); alloy: AA3003, both sides clad 10% AA4343 and angles; alloy AA1050. The angles were put on the coupons and some flux paste was applied at the joint area. About two droplets of the paste were applied on each coupon. First set of coupons and angles assemblies was kept in laboratory at ambient temperature and normal humidity conditions. The paste dried out during storage.
Second set of coupons and angles assemblies were stored in a closed plastic containers.Firstly a paper tissue saturated with demineralized water was put at the bottom of the plastic container and then the coupons with angles were placed on top of the paper. There was no direct contact of the flux paste with water saturated paper. After closing the containers they were placed on a standard room heating radiator (temperature around 40 °C to 50 °C) for 2 days, then stored at room temperature for 14 days (~ 22 °C), and again placed on the radiator for 4 days.
After the total time of 18 days the assemblies were removed from the containers, photographed and subsequently dried at ambient temperature. At the end all of the assemblies were washed with demineralized water, and the paste was wiped off by the use of a paper tissue.
Fig. 3: Fluxed assemblies after 18 days.
Left: Storage in open air, right: Storage in humid conditions
Fig. 4: Rinsed and wiped off coupons after 18 days of storage.
Left: Storage in open air, right: Storage in humid conditions
As can be seen from the pictures above, storing of fluxed coupons at standard dry condition did not cause any interaction with aluminum substrate. Storing at humid and slightly higher temperature caused corrosion of aluminum substrate.
In order to check the kinetics of water absorption by the flux and glycol based flux paste another experiment was performed. The experiments were conducted in a lid closed bucket. At the bottom of the bucket there was a layer of demineralized water and the wall of the bucket was covered with absorbing paper which due to the contact with the water at the bottom was soaked all over. The experiments were done at room temperature (20 – 22 °C). Small samples of NOCOLOK® Flux powder and flux paste were put into aluminium dishes making sure that the flux did not have a direct contact neither with water nor with the soak paper. The samples were kept in the buckets for 12 days and the weight gain was measured on regular basis.
Fig. 5: Weight gain of glycol based flux paste and NOCOLOK® Flux powder kept in humid environment (bucket)
The water take-up curve of the NOCOLOK® Flux powder sample shows the usual behaviour where there is a minimal weight gain due to due to hydration of K₂AlF₅. In contrary the sample of glycol based flux paste shows excessive water take-up. It can be explained by the well known fact that glycols are hygroscopic.
Summary and conclusion
The above experiments show that when dry NOCOLOK® Flux powder is in contact with aluminium there is no interaction between the flux and the substrate; however when there is some water present, aluminium starts relatively quickly corroding. Therefore storage of assemblies with applied glycol based paste; at high humidity conditions must be avoided.
Complete drying of glycol-based flux pastes avoids the risk of corrosion.
Hinterlasse einen Kommentar
An der Diskussion beteiligen?Hinterlasse uns deinen Kommentar!