Yearbook 2013/2014 - ehedg

New developments for upgrading stainless steel to improve corrosion resistance and increase equipment hygiene 125
New methods. Unlike the conventional method, new
methods have been developed to produce the desired
level of corrosion resistance by changing the consistency
and structure of existing passive layers on stainless steel,
independently from the alloy and structure of the metallic
base. These methods—one chemical and one thermal—are
applied as final treatments after fabrication and substantially
increase corrosion resistance.
Chemical treatment. A precondition for the application of
the new chemical treatment method is that the stainless
steel to which it is applied must have an existing passive
layer. Therefore, its application immediately following a
pickling process is not effective.
The chemical treatment selectively breaks the iron oxides
within passive layers and extracts the iron without affecting
or removing the passive layer. In this way, the concentration
of iron in passive layers is strongly reduced and the chrome/
iron ratio is substantially increased up to values of 6 to 8 (Fig.
1). This treatment of stainless steel substantially increases
the resistance to all types of corrosion (Figures 2 and 3). The
resistance to thermal discolouration is raised to 100-150°C.
Fig. 2. Structure of passive layer on stainless steel AISI 316 Ti -
original condition.
Fig. 3. Structure of passive layer on stainless steel AISI 316 Ti –
chemically treated.
Fig. 1. The chemical treatment significantly reduces the
concentration of iron in passive layers and the chrome/iron ratio is
substantially increased up to values of 6 to 8.
The applied chemicals are water-based solutions of organic
and biologically degradable substances, mainly comprised
of a special combination of chelating and complexing
agents. They do not contain mineral acids or their salts and
have a pH value of about 4.0. Application does not produce
harmful fumes or foul odours. Since no dissolution of metal
or passive layers takes place, the liquid does not contain
heavy metals in noticeable concentrations.
Application can be done by dipping, spraying or wiping during
a three- to four-hour period. The temperature in dipping tanks
should be kept above a minimum of 50°C to avoid biological
degradation. Higher temperatures increase the effect of the
treatment, while longer treatment times do not. All types of
finishes and nearly all types of stainless steel can be treated.
However, when the chrome content in the alloy is less than
15% the required temperature, concentration and time of
treatment must be modified.
Thermal treatment. The effect of chemical treatment can
strongly be increased by a subsequent controlled-heat
treatment. The heat treatment optimises the structure and
distribution of elements in the passive layer and increases
its thickness. The thermal treatment leads to the formation of
a second layer containing iron oxides on top of the existing
passive layer mainly formed by chrome oxides. These layers
are semiconductors forming a n/p-transition and immediately
provide a further substantial increase in corrosion resistance
(Fig. 4).
The heat treatment takes place under atmospheric conditions
at temperatures in the range of 120-220°C, dependant on
the alloy, and for a time of 5 to 10 minutes.