Hello, everyone. Last month, I began a five-part discussion on the metals used to fabricate spray packages and packaging components (I added another part to include laminated metals). The corrosion resistance for the different packaging metals is significantly different.
This month, we’ll continue with a discussion on tinplated steel, commonly referred to as tinplate or ETP (electroplated tin).
Tinplated Steel container metallurgy
Table 1 lists the compositions of the most common steel alloys used to fabricate tinplated steel sheet used for ETP aerosol containers.
Maximum percent in the steel alloy
|Element||Type D||Type L||Type MR|
|Iron||(Balance to make 100%)||(Balance to make 100%)||(Balance to make 100%)|
|Other elements (not specified)||0.02||0.02||0.02|
Table 1: Chemical Requirements for Tin Mill Products
(ASTM specification A623-02a)
The various elements listed in Table 1 are dissolved in iron to make it steel, increase the strength and formability of the steel and to enhance the steel-making process. For example, aluminum and silicon are added to react with air entrained in molten steel to prevent violent bubbling of the molten steel when hot air escapes from the molten metal—referred to as killing the steel
Notice in Table 1 that the three container steels have small amounts of chromium and nickel. I was told when I first started working on spray package (aerosol) corrosion that the chromium and nickel were added to give the steel corrosion resistance. Nothing is further from the truth! Chromium and nickel are added to steel to make the sheet metal smooth for lithography and labels.
Steel, like aluminum, needs a protective surface coating
As a general rule, the corrosion resistance for pure iron is typically higher than the corrosion resistances of the steel alloys listed in Table 1. Indeed, the three steels in Table 1 would rust within a few hours of exposure to a humid summer day without some kind of surface protection.
Consequently, the steel for aerosol containers have a thin coating of tin on both sides as illustrated in Figure 1. The tin thickness can be either the same on both sides or thicker inside the container than on the outside.
Figure 1 Units: 1 mil = 0.001 inch; 1 micron is approximately 3.9×10-5 inch; 1 nanometer is approximately 3.9×10-8 inch.
A thin interfacial layer of FeSn2 iron-tin alloy forms between the base steel and the tin metal during tin electroplating on steel. Heating after electroplating typically thickens the iron-tin alloy on the ETP sheet and gives its surface a bright finish instead of the natural matte finish.
In the early 1980s, it was hypothesized that the iron-tin alloy layer provided all of the corrosion protection for tinplate containers (referred to as K-plate). However, many corrosion tests invalidated this hypothesis.
Tin corrodes in air and forms a very thin mixture of tin hydroxide and tin oxide on top of the metallic tin. The chromium/chromium oxide layer is formed when ETP is rinsed with a chromium solution.
A thin layer of food-grade oil is often sprayed on the finished tinplate to help prevent further atmospheric corrosion of the tin coating. The oil is also usually compatible with container coatings.
The tin coating protects the container steel from atmospheric rusting while containers are stored prior to filling. However, tin coatings do not always protect the container steel from corrosion by formulas. Indeed, the tin coating is very thin and is often rapidly removed by corrosive formulas, thereby exposing the base steel to corrosion.
The corrosion resistance of ETP is determined by the chemical composition of a formula. Consequently, the only way to determine if aluminum containers are suitable for use with a given formula is, once again, to conduct corrosion tests. Corrosion tests could be either a traditional storage stability test (static storage test) or the shorter electrochemical corrosion test.
Next issue, we’ll continue with a discussion of tin-free-steel (TFS).
Pair O Docs has a state-of-the art electrochemical corrosion testing laboratory; please contact me if you would like to know more about our faster and predictive corrosion testing. You can also visit our new website which has a short Vision Video that discusses all our corrosion prevention and control services. Please also contact me if you would like to a have our Elements of Spray Package Corrosion short course taught at your R&D facility. Thank you for reading Corrosion Corner and I’ll see you in November. SPRAY