Nowadays there is a growing demand for gas cylinders of high purity and stability for high-purity special gas mixtures. Recognizing the need and with an experience of more than 30 years, we've developed a new procedure for gas cylinders' inner surface treatment, which can ensure a more efficient surface for high-purity gases than any previous solution.
How does the procedure we developed differ from existing procedures?
Basically, there were 3 different surface treatment procedures available on the market:
- Surface treatment using gases, gas mixtures
- Electropolishing with acid solutions
- Application of inner protective coating, e.g., Teflon
During preliminary experiments we've thoroughly examined the surface quality offered by the previous procedures and subsequently we've developed a procedure, which substantially reduces surface roughness and thus increases the gas stability of cylinders.
The developed new procedure is combining several methods to profit from their advantages, but to eliminate their disadvantages.
What should we know about surface roughness?
Even if it looks smooth, a surface examined under a microscope is rough, which means that peaks alternate with valleys (see figure). The average value of roughness measured on a given section represents the average of the surface roughness (Ra).
The distance from the highest peak to the lowest valley represents the maximum difference (difference min-max). The average difference of the peaks and the valleys is the (...)
Maybe these are the 3 most relevant indicators that you need to consider when examining the roughness of a surface.
In terms of gases, gas mixtures, the rougher the surface and the bigger the MIN-MAX difference is, the less the cylinder can keep stable the filled gas mixture.
How to check surface roughness?
The surface roughness was measured with an Ambios XP-1 profiler device. We used gas cylinders of the same size, the same material and from the same manufacturer. We cut open the treated and the untreated cylinders, and we made measurements in 5 different points of the cylinders inner surface, using the average values in this article.
What are the methods combined by the new procedure and why?
Different procedures and different factors used during the procedure:
- Corrosive liquid - acids
Abrasive materials
Abrasion procedure
Abrasion time
Acidification time, treating time
Finishing, cleaning and protection
The developed procedure is an appropriate combination of the above factors. Acids have a violent reaction when they come into contact with aluminium. They can dissolve layers of many microns from the surface of the cylinder, but they not only dissolve the peaks, they also deepen the valleys. The procedure improves the average surface roughness of the cylinder, but it often degrades the MIN-MAX values, as well as the MIN-MAX difference, which represents an important aspect of gas stability. (Figure)
The new procedure uses a smaller amount of acid, which triggers the process and dissolves the peaks, but combines it with mechanical polishing by aluminium bead blasting. The latter helps removing the peaks, but it does not entail any substantial valley deepening. With the right combination of the two procedures, it can be obtained a more efficient abrasion as previously to leave uniquely smooth surfaces. The smoothness of the surfaces may be controlled by changing the duration of treatment. The procedure offers surface roughness improvement from 50% up to 200% or more.
As you can see, the procedures which only use the polishing technique with acid solutions may lower the peaks, but they also deepen the valleys, thus the result won’t better in all parameters than the original factory surface. Our new procedure which combines different techniques can significantly improve all values, so actually we can obtain a smoother inner cylinder surface and a HIGHER GAS STABILITY.