Stainless Steel and Hygienic Design
Claus Qvist Jessen & Erik-Ole Jensen; Damstahl a/s
Everybody wants to stay healthy, and food hygiene
is an integral part of this. Outside Europe, the lack
of fridges and the use of wooden cutting tables frequently causes stomach illness among tourists. In
Europe, such matters are easily taken care of in our
kitchens, however, when the food/pharma moves
into large-scale plants, all cleaning has to be carried
out automatically. As we are suddenly not able to
see what we are doing, the risk of contamination is
far higher, increasing the risk of severe illness and,
at worst, death. That this is not all fiction was
proved in Denmark in the summer of 2014. Here, an
inadequate cleaning procedure at a factory producing pork rolls caused a major outbreak of Listeria
which in turn caused the tragic death of 18 people.
In 2015, Listeria caused another three deaths in
Denmark.
Fig 1:
Further south, keeping the meat in the sun, directly
exposed to the heat of the sun as well as the plethora of flies, frequently causes a few upset stomachs. This happy butcher is from Mauritania, West
Africa; note the swarm of flies on the piece of meat
to the lower left.
Stainless Steel is not Always Stainless
On the micro level, the cleanability of the stainless
steels surface is closely related to the corrosion resistance of the steel.
Stainless steel owes it’s fantastic corrosion resistance to an invisible layer of mainly chromium oxide. This “passive” layer effectively protects the
metal and ensures that the corrosion process never
really gets any further than the natural maintenance
of the oxide layer. Treated correctly, and not exposed to extreme conditions, stainless steel becomes
“an eternity metal”. If stressed beyond its capability,
corrosion may occur very fast, and penetration may
be a question of weeks rather than years. Either the
steel remains passive and lasts forever, or failure
happens within a few weeks of operation.
In that respect stainless steel reminds a lot of a fishing line (Fig. 2)! If a fishing line is subject to a load
below its breaking strain, it lasts forever, however,
if the load is too heavy, the line snaps at once. Most
anglers know the sad feeling of losing river giant
due to a weak line.
Stainless steel behaves very much the same way.
When subject to a load below the breaking strain,
the system lasts forever – even though we get very
close to the limit, such as a 17.9 kg load in an 18 kg
fishing line. Once the load exceeds the breaking
strain of the line, the punishment is swift and consequent (Fig. 2). The fishing line metaphor even explains the importance of correct manufacturing.
Heat tintings, surface defects, incorrect welding,
crevices and other weaknesses may reduce the
breaking strain of the line, and corrosion may occur
at conditions which should be harmless when exposed to the perfect fishing line. Actually, we can
learn a lot about the corrosion properties of stainless
steel by going fishing……. ☺
consideration – not the distance in between the
peaks. This is very important as the corrosion resistance as well as the cleanability of the surface are
highly dependent on the surface topography. Thus,
rather than trying to describe the surface condition
through just one single number (Ra), we recommend
that the surface profile is used. However, evaluating
from the profile whether the surface is good enough
or not is much more difficult.
Fig 2:
Corrosion of stainless steel frequently resembles a
fishing line. If the line is stressed below its breaking strain, it lasts forever. Alternatively, if the line
is stressed beyond the limit, failure occurs quickly.
Here, increasing the load from 10 to 17.9 kgs does
not affect the life-time of fishing line (with 18.0 kg
breaking strain). Increasing the load from 17.9 to
18.1 kg causes the line to snap. There is nothing in
between.
In practice, pitting corrosion, crevice corrosion and
stress corrosion cracking (Fig. 3) are all major dangers to the stainless steel equipment, and the risk of
corrosion has not reduced throughout the years. This
is due to the fact that the production time of the
equipment has increased from, say, eight hours daily to more than 16 hours. As such, the cleaning time
is vastly reduced, and stronger and stringer disinfectants are used. Consequently, the “standard stainless steel grade” in food/pharma has moved from
the 4301 class (~ AISI 304) to the molybdenumcontaining 4401 class (~ AISI 316).
Different Surface, Different Ra
Both dealing with corrosion and hygiene, the surface of the steel plays a major role. As a rule, a fine
surface has a tendency of increasing the corrosion
resistance as well as the cleanability, and this has
made various authorities to implement a “standard”
saying that Ra ≤ 0.8 µm.
In practice, Ra is an average throughout a certain
length, however, it only takes the heights of the
roughness peaks and the depth of the valleys into
Fig 3:
A fine example of stress corrosion cracking in
austenitic stainless steel type EN 1.4307 (AISI
304L). The steel originates from the bottom of an
oven used for baking bread, and the corrosion
was caused by salty water dripping onto the bottom of the oven.
As such, using a single factor like the Ra has a certain purpose, however, it is only useful for comparing two surfaces which have been subject to similar
treatments – i.e. a ground surface with another
ground one, or two rolled ones. In contrast, the Ra is
less useful when comparing two specimens which
have been treated differently.
By experience, a ground surface of Ra = 0.8 µm is
somewhat less corrosion resistant and equally less
cleanable than a cold-rolled surface sporting the
same Ra. Then why doesn’t this always show in a
corrosion testing? Because corrosion testing is usually done with respect to pitting corrosion alone,
and such a testing only lasts a couple of hours. In
such a short time span, crevice corrosion may not
reveal itself as a threat, and a surface containing
crevices may show a too positive result. Such a
surface is shown below in Fig. 4.
Doing so, a higher Ra is accepted for the cold-rolled
2B, simply because such a surface is, by experience,
easier to clean (and is more corrosion resistant) than
a ground surface with the same Ra.
Ground / glass bead-blasted: Ra ≤ 0.8 µm
Fig 4:
“Real” micro section through a ground (P80)
surface, 4404, 2 mm. The yellow line shows the
real surface. The blue line is the roughness profile when using indefinitely fine laser equipment;
the red one show the profile when using a physical roughness meter. The two green arrows show
“hiding places” for bacteria = initiation sites of
crevice corrosion. Such sites can never be measured properly. Photo by Rasmus Lage; Technical
University of Denmark.
All this is particularly important above water (or
periodically above water), as the various types of
surface vary a lot when it comes to holding on to the
corrosive/bacterial water. After all, there is a reason
why we grind our garage door before painting it:
We want the new paint to stick – and so does the
saltwater, the dirt and other corrosive stuff, along
with bacteria and other contaminations. Thereby, it
is obvious that the almost universal Ra ≤ 0.8 µm is
far from ideal and doesn’t really relate to “real life”.
A fine example of the importance of the surface
treatment when dealing with contact time is shown
in Fig. 5.
Ideally, the Ra should reflect the practical experience, and we therefore recommend that different
Ra’s are used for different surface classes, all dependent on the mechanical surface treatment.
2B, cold-rolled:
Ra ≤ 1.2 µm
2B, post-pickled:
Ra ≤ 1.2 µm
Weld, pickled:
Ra ≤ 3.0 µm
Such a list is never perfect, however, it correlates
with practice. As such, there is no reason to grind
the tank welds at all. In practice, a “perfect” weld is
quite corrosion resistant and equally cleanable. After all, all assembly welds in tube systems are left as
they are (pickled at the most), strongly indicating
that there is no need for a subsequent grinding. If
needed, the tank welds may be ground in a narrow
belt, 40 mms on each side of the seam.
Fig 5:
Superficial pitting corrosion above water observed
on a stainless steel tube grade 4432 (2.5 % Mo)
mounted of a salt vessel at a Danish dairy. The
back wall suffers from no corrosion at all, despite
the fact that the conditions are exactly the same as
the tube – and the wall is made of the lower-alloyed 4404. So, why the difference? The tube has been
ground, while the back wall hasremained as the
dull and grey 2B. The latter is much less prone to
collecting salts and humidity than any ground or
blasted surface.
Where to Find a 30 cm Tall Welder?
During the latest decades, outsourcing has become
increasingly popular. Unfortunately, this is bad
news for the communication in between the designer and the welder. The larger the distance in between the two (physically and mentally!), the bigger
is the risk of a communication breakdown.
Through blueprints, the orders may flow from the
designer in Europe to the welder in China, however,
the designer never gets to know if his design is impossible to make properly. Often his design includes
very little space for the welder to do his job properly, however, there is no one to tell him that the
30 cm welder has yet to be born.
Fig 7:
The example from Fig. 6 carried out in practice.
Flow Design
Ideally, flow systems should provide the free and
unhindered flow from A to B, including a high degree of drainability, and a minimised risk of creating air pockets. Some general “advice” is listed
below:
Ensure that the equipment can be properly
cleaned.
The simpler, the better.
Drainability! Stagnant water may end up as
a puddle of bacteria, and may cause corrosion as well.
Corrosion may lead to hygienic problems.
In order to deliver the best performance, the
designer and constructor need to know the
production methods. Unfortunately, this is
often classified as “a company secret”.
Pick the best surface for the job – not necessarily the one with the lowest Ra (see
above).
Optimize the welding procedure; no weld
defects are permitted – crevice corrosion
and bacterial growth may take place.
Good manufacturing practice, please.
Fig 6:
Not all designs are equally easy to make. On the
paper, the grey top version (30 x 15 mm steel
bars, 4307) looked brilliant, however, welding
the 30° angle proved close to impossible due to
the lack of physical space. Instead, inserting a
bend in the lower bar (green drawing, below)
made the whole construction possible.
Fig. 6 and Fig. 7 show such an example. The 30°
angle on the top drawing may look good on the
blueprint, however, making the weld requires a very
small welder with an equally small piece of equipment. Such a message never gets back to the designer. Instead, by inserting a bend on the lower bar
(the green drawing below) the construction is easier
to make and more easy to keep clean.
However, even obeying all these “rules”, funny
things may happen as shown in Fig. 8.
Here, the designer obviously loves elbows, as his
design includes no less than four extra pieces of
fittings. From a supplier point of view, this may be
close to perfect, but from a company point of view,
this is unattractive for at least four reasons:
Extra cost for buying the four elbows
Extra cost for the assembly (five or six extra
welds)
Increased pressure loss during operation =
extra costs
Increased risk of air pockets in the top of
the loop
Instead of the present design, it would have been
much better to skip the four elbows and instead
move the T at the blue arrow to the position of the
green arrow. Case closed!
Fig 8:
Fittings are necessary, however, they should like
anything else be used with thought and care.
Here, no less than four extra elbows have been
inserted into the construction (the red arrows),
however, that solution is bound to increase the
costs of building and operating the system. By
moving the T-piece from the position of the blue
arrow to the green one, all four elbows can be
omitted.
Claus Qvist Jessen is chemical engineer (MSc) and
PhD and works with Damstahl a/s, Denmark, as a
consultant engineer specialised in stainless steel,
corrosion and corrosion As a writer Claus has produced a number of articles on the topic, and left his
footprint in several articles as well as five books on
the topic, notably “Stainless Steel and Corrosion”
(Damstahl, 2011) and “Stainless Steel for Hygienic
Design in Food / Pharma” (Damstahl, 2015).
Erik-Ole Jensen is mechanical engineer (BSc) and
has spent more than 25 years in the Danish food
industry, mostly with Arla. Currently, he is employed with Damstahl a/s as consultant engineer,
specialised in hygienic design, control and inspection. As a writer, Erik-Ole has produced a number
of articles as well as half of the book “Stainless
Steel for Hygienic Design in Food / Pharma” (Damstahl, 2015).