W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
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.
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 deter-
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.
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.
30 Spray October 2017
The only way to determine if aluminum
containers are suitable for use with
a given formula is, once again, to
conduct corrosion tests...
Corrosion Corner
Spray Package Metallurgy and
Corrosion—Part II : Tinplated Steel
Maximum Percent in the Steel Alloy
Element Type D Type L Type MR
(Balance to (Balance to (Balance to
Iron
make 100%) make 100%) make 100%)
Carbon 0.12 0.13 0.13
Manganese 0.60 0.60 0.60
Phosphorous 0.020 0.015 0.020
Sulfur 0.03 0.03 0.03
Silicon 0.020 0.020 0.020
Copper 0.20 0.06 0.20
Nickel 0.15 0.04 0.15
Chromium 0.10 0.06 0.10
Molybdenum 0.05 0.05 0.05
Aluminum 0.20 0.10 0.20
Other elements 0.02 0.02 0.02
(not specified)
Table 1: Chemical Requirements for Tin Mill Products
(ASTM specification A623-02a)