The metals used to fabricate spray packages include:
- Steel alloys (both tinplated and tin-free)
- Aluminum alloys
- Laminated aluminum alloy foils
- Stainless steel alloys
Stainless steel is the least visible of all packaging materials, but is used in all types of spray packaging. Stainless steel is used to fabricate the springs in spray package valves and the check balls in some types of valves.
Figure 1 shows a cutaway of a spray package valve. The cutting process slightly distorted the spring, which would not normally be so affected.
The most common type of stainless steel is 304 (UNS S30400), which is composed of 18% chromium and 8% nickel in the alloy; trace amounts of other alloying elements (e.g., carbon) with iron making up the balance. In other words, stainless steel is an iron alloy with chromium and nickel as the major alloying elements.
The presence of chromium and nickel in stainless steel gives this iron alloy a unique surface property—the ability to form a very thin, complex chromium oxide/nickel oxide barrier layer or film. This oxide film protects the underlying metal from corrosion by a wide variety of environments, such as consumer formulas.
Please note that the steel alloys and aluminum alloys also used to fabricate spray packages do not have the requisite amount of chromium and nickel in them. Consequently, spray package steel alloys and aluminum alloys do not form a protective surface barrier film of chromium oxide with nickel oxide.
The mechanism by which corrosion and pitting corrosion attacks stainless steel is significantly different from the mechanism for corrosion of other spray package materials. There are several competing theories on how stainless steel pitting corrosion occurs. Each theory has its merits, but the ion exchange mechanism is the most plausible theory to this author.
Halogen ions, such as chloride ions, are more electronegative than the oxygen ions in the chromium-nickel oxide barrier film on stainless steel. Consequently, halogen ions are able to displace the oxygen ions in the oxide barrier film and form chromium halogen/nickel halogen complexes.
These metal ion halogen complexes are water soluble and thus removed from the barrier film by dissolution in the formula water or contaminant water. Removal of portions of the film causes localized damage to it. However, the film is able to repair itself as long as the rate of repair is greater than the rate of damage and other chemicals in the environment, such as low pH (i.e., a large number of hydrogen ions) does not prevent the film from repairing itself.
Stainless steel pitting corrosion initiates and propagates when the damage by halogen ions exceeds the rate at which the film is repaired, causing holes in the film that develop into pitting corrosion.
The mechanism for stainless steel corrosion by halogen ions has somehow been erroneously applied to the steel and aluminum alloys used for spray packages. However, the halogen mechanism is not applicable to these two alloys because they do not form chromium-oxide/nickel-oxide surface barrier films.
In addition, the steel alloys used for spray packaging contain amounts of chromium and nickel to control grain size and prevent the surface of the steel from looking like an orange peel. However, the amount of chromium and nickel in these steels is significantly below the 12% minimum needed to form the protective barrier oxide found on stainless steels.
I have observed a few instances in which valve springs corroded and broke when exposed to low pH formulas. However, the instances of valve spring corrosion and failure with other consumer products have been rare, making stainless steel the material of choice for valve springs.
I advise removing and inspecting valve springs during storage stability corrosion tests, just to be on the safe side. Inspection of valve springs should not add a significant amount of time to a normal corrosion examination. Electrochemical corrosion tests can also be used to confirm that the stainless steel valve springs are not corroded by a given formula.
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