Yearbook 2013/2014 - ehedg

European Hygienic Engineering & Design Group
New developments for upgrading stainless steel to
improve corrosion resistance and increase equipment
hygiene
Siegfried Piesslinger-Schweiger, POLIGRAT GmbH, 81805 Munich, Germany, e-mail: petra.ressmann@poligrat.de,
www.poligrat.de
New developments enable the upgrading of stainless steel to
improve the corrosion resistance of manufacturing equipment
and components. Unlike the state-of-art techniques that use
higher alloyed steel to produce corrosion-resistant equipment
and components, new methods have been developed
that can be applied as final treatments after production
and elevate the passive layers independently from the
underlying metallic base. One of these methods is based
on a significant increase of the chrome/iron ratio within the
passive layers by extraction of iron and iron oxides, leaving
primarily chrome oxide. The second is a heat treatment that
changes the structure and thickness of the passive layer.
The latter application can be utilised on all types of finishes
and in nearly all commonly used alloys.
These new methods result in a substantial increase of the
resistance against any type of corrosion. They also allow an
effective restoration of corroded surfaces, and with regular
application, can maintain corrosion resistance even in
cases in which stainless steel is not long-term resistant. The
treatments also can be applied to scale and heat discoloration
without pre-treatment, which could widely replace pickling or
mechanical descaling.
Since these methods are based on a treatment with a waterbased
solution of special organic compounds, they are
biodegradable, environmentally friendly, and produce no
fumes or nasty smells. The new methods also allow selection
of the best alloy and structure in terms of hardness, strength
and weight. They open a wide and commercially important
potential for additional applications of stainless steel.
Basics of corrosion-resistant stainless steel
According to the state of the art, the corrosion resistance
of stainless steel is considered a secondary property
of alloy and structure. To increase corrosion resistance
it is necessary to select a higher alloy quality. To meet
the objectives of development a fundamentally different
approach to stainless steel and its functional behaviour is
necessary.
Stainless steel is a composite consisting of a metallic base
and an oxidic cover layer, and the passive layer is similar to
aluminium and titanium. The metallic base determines the
material’s mechanical, electric and magnetic properties and
provides the metals for the formation of the passive layer.
The passive layer determines most of its other properties,
including corrosion resistance. As soon as passive layers
are locally damaged, local corrosion of the metallic base
occurs, such as pitting corrosion, crevice corrosion, Ironinduced
corrosion, stress corrosion cracking (SCC), and
more.
Passive layers completely and densely cover the surface
of stainless steel as long as it does not corrode. Passive
layers are 10 to 15 nm thick and are formed by the reaction
of the metallic base with oxygen from the environment.
They primarily consist of chrome oxides and iron oxides.
Additionally, they contain metallic chrome and iron, and
eventually, other metals like nickel and molybdenum.
Passive layers on stainless steel are not insulators like
the oxides on aluminium and titanium. They are crystalline
semiconductors with all the special properties of these
materials. Thus, the approach to understanding corrosion on
stainless steel should include semiconductor physics.
The ratio of chrome oxides to iron oxides (chrome/iron
ratio) typically is within the range of 0.8 to 2.0. The higher
this ratio, the better the corrosion resistance. That means,
that chrome oxides increase and iron oxides reduce the
corrosion resistance.
Methods to increase corrosion resistance
To improve the corrosion resistance of stainless steel two
methods are promising success. The first is to improve the
chrome/iron ratio within the passive layer and the second is
to improve the crystalline structure.
Conventional state-of-art method. According to the current
state-of-the-art approach, the method for raising the chrometo-iron
ratio within passive layers consists of reducing the
concentration of iron in the metallic base and increasing the
concentration of chrome, and eventually nickel, in the alloy.
This secondary effect leads to a higher chrome/iron ratio
in the passive layers. The structure of passive layers is not
influenced. The concentration of alloying elements besides
iron is only needed within a surface layer of less than 10-
nm thickness to provide the metal for the formation of the
passive layer.
There are a few downsides to the conventional method of
producing corrosion-resistant stainless steel. The adaption
of total alloy and structure to form the passive layer
substantially determines the other properties of the alloy.
A potential consequence of this is that expensive details
of construction like wall thickness and weldability must be
adapted. The adaption of alloy and structure to achieve
gains in the level of corrosion resistance can only occur in
the production of steel. This means that a great number of
qualities of stainless steel must be produced and be available
as semi-finished product, which reduces the flexibility in the
material’s application and increases costs.