W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
Corrosion Corner
Spray Package Metallurgy and
Corrosion—Part III : Tin-free Steel
Hello, everyone. Last month we had Part II of a five-part series
on the metals used to fabricate spray packaging. This
month, we continue with a discussion of tin-free-steel,
more commonly referred to as TFS.
Sheets of TFS have a similar chemical composition to that for
the steel used to make ETP (tinplated steel). However, the surface
treatment for TFS is much simpler, as shown by the diagram in
Figure 1. Notice that the surface of TFS is essentially only one
layer of a very thin mixture of metallic chromium and chromium
oxide.
TFS is commonly used to fabricate food containers, but there
are steel aerosol containers that are also fabricated with TFS.
There is an old axiom that says polymer coatings bond more
strongly to chromium than to tin. However, I’ve not found any
referenced technical publications that either prove or disprove
this saying.
The corrosion rate-pH behavior for both TFS and ETP are
determined by the base steel and chemical composition of a
formula. Figure 2 provides a corrosion rate graph for steel as a
function of pH magnitude. This graph is a very old one and was
generated for a steel alloy similar to that used for aerosol containers.
The solution for this graph is water.
Figure 2: Steel corrosion rates as a function of solution pH
38 Spray November 2017
Chromium-chromium oxide
≥ 1 nanometers thick
Steel thickness ranges
from 6 to 8 mils
Figure 1: Tin free steel structure