Electrolux AOS101GTD1 User Manual

FOOD SERVICE EQUIPMENT

Technical Bulletin

Page 1

Identification:
Bulletin no:
Date:
Issuer:
Factory:

English language

GENERAL

PDX2000-09

19-09-2000
R.ZAGO (T.T.&S.)

Vallenoncello Cooking

THE WATER AND THE APPLIANCES FOR

PROFESSIONAL COOKING

ELECTROLUX FSE Electrolux ZGI V.le Treviso 15 [phone] [phone]

33170 PORDENONE - ITALIA

POSTAL ADDRESS TELEPHONE TELEFAX

THE WATER AND THE APPLIANCES FOR PROFESSIONAL
COOKING

WHAT YOU NEED TO KNOW BEFORE SELLING AND INSTALLING AN APPLIANCE

Object

The lack of information, and the fact that there is not such a thing as a “culture of the water” among
those professionals who operate in the catering sector, is most times the source of many technical
problems which then cause the fast degradation of performance and reliability of the appliances used
for cooking foodstuff. The object of this technical bulletin is that of supplying our retailers and
engineers who carry out the installation and servicing of machines and appliances with the necessary
and sufficient elements that will allow them to evaluate first and to operate then so to avoid any
damages the water might provoke (corrosion and scaling) on the materials used for manufacturing the
machines and to choose the best water treatment possible.

Application field

Combined ovens with and without steam generators, indirect pans and all other functions of usual
modular catering .

The water, general news …

The water in the world is ever the same, it gets continually recycled in a perfect closed system. The
water in the seas, rivers and lakes evaporates continuously provoking the separation of impurities and
salt which remain in solid state.
Therefore water ascend towards the higher stratus of the atmosphere where it condenses to the solid
state dissolving (loading) the oxygen and the nitrogen and then precipitates on the earth (rain, snow,
hail) dissolving the carbon dioxide that the water contains. When the water enters in contact with the
soil and while flowing through the superficial stratum, the salt contained in the water dissolves. Then
water flows along impervious underground stratum until it gets pumped outside or it finds a natural
outlet. Now it continues flowing, it loads bacteria, it fills with vegetal and animal life until it evaporates
again so starting its natural cycle.

a) The water may contain substances such as:
b) Big suspended particles (sand, organic substances)
c) Average colloids (sulphides, metallic hydratez, detergents)
d) Molecular solutes such as gas, undissociated (sugar and silicates) and dissociated (ions)

molecules.

The main gas dissolved in water are… … Nitrogen (N2), Oxygen (02), Carbon dioxide (CO2),
Ammonia (NH3), Natural gas (CH4), Chlorine (CL2).
And the ions dissolved in water are …. Hydrogen (H+), Potassium (K+), Calcium (Ca++), Magnesium
(Mg++), Iron (Fe++), Copper (Cu++), Chloride (Cl-), Bicarbonate (HCO3-), Carbonate (CO3--),
Sulphate (SO4--), Phosphate (PO4--)

Apart from these elements the water contains also living organisms such as mildew, weeds, bacteria,
virus and chemical compounds deriving from industrial technological treatments.
Water is considered drinkable when suitable for human diet. To be drinkable water must contain every
element within a maximum limit of concentration.
Other substances must be absolutely absent such as: bacteria, virus, mildew, weeds, fungus, lead,
arsenic, chromium, cadmium, barium, cyanide, selenium.
Drinkable water might not be suitable for the correct functioning of a cooking appliance.

What is the corrosion…

Corrosion is the electrochemical phenomenon that provokes the progressive deterioration of all metallic
materials that enter in contact with aggressive agents (water, humidity). All professional cooking
machines are made of stainless steel (Aisi 304, 430); these materials are more resistant to corrosion
than other ones thanks to the greater presence of chromium (10,5 that distinguish stainless steel from
other iron alloys. This element is capable or reacting (passivation) when entering in contact with air
containing great quantities of oxygen (oxidant agent) by forming a special oxidized superficial film that
protects the material underneath. When stainless steel is in these conditions it is called “passive” and it
means that the metal is subjected to a very slow thermodynamic corrosion. Otherwise stainless steel is
called “active” when it reacts rapidly to the corrosion process. Naturally the passive film will get
recreated when ripped if the environment is sufficiently oxidant and if the oxygen is able to enter in
contact with the alloy. In short, it means that these materials protect themselves against the corrosion
by means of an “auto-regenerative protective skin” that slows down the electrochemical corrosion
process. According to the state it is in, passive or active, the stainless steel will oppose a valid
resistance to corrosion or will be destroyed by it. Even if the stainless steel resistance to corrosion is
greater that that of other metal it does not mean that, depending on its atomic structure, on special
environmental conditions, on the type of technological process undergone and mainly to the presence
of particular aggressive agents, after a variable incubation period the material protective film might get
ripped. In these conditions the destruction of the material underneath is very fast arriving to break or
even to get perforated. Hence there are not such a thing as a “magical” material but only materials that
can be more or less resistant to corrosion. There are various forms of corrosion, the following ones are
those more common for cooking appliances:

UNIFORM CORROSION

It is most common for fry-top and iron brat-pans. Corrosion forms a oxide film on a wide surface when
in presence of a very aggressive environment (humidity, water), it attacks mainly iron and specially
rough surfaces.

STRESS CORROSION Fig. A

Fig. A

It is very common on cooking appliances and it shows on
cells, steam generators and oven heat exchangers and in the
indirect pans cavity walls. It appears when the material present
in the corrosive environment gets mechanically stressed by
traction or cut in static or dynamic conditions. It takes the
form of branched cracks near the welding at a very high
penetration speed (up to 2 mm/hour) so that when the crack
becomes visible the material is already damaged. Stress
corrosion happens mainly caused for the following connected
facts:

• Stress by external traction due to the use of the appliance
itself or caused by the presence of material internal pressures
due to cool mechanical process (plastic deformation such as
stuffing and bending) or to thermal treatment (welding).

• Presence of an corrosive environment ( Chloride )

• Frequent thermal variations and high temperature.

Stress corrosion is a electrochemical corrosion but accelerated by the mechanical stress.

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Intercrystalline corrosion Fig. B

Fig. B

It etches chromium stainless steel (Aisi
430-304-316-321), the material crumbles
and gets pulverized grain by grain. The
crystal lattice that unites the molecular
structure of stainless steel brakes due to
the precipitation of the chromium carbide
on the borders of grains at temperature
higher than 500°C. Imagine 2 contiguous
crystals, part of the chromium migrates
towards the border between two grain so
incrasing the concentration of chromium
on the border of every grain which
becomes much higher than the material
average. This way the concentration on
zones (fig. B1) near by the grains borders
gets depleted of chromium until
“uncovering” the zones right by the

borders that this way become vulnerable to
corrosion agents. So both grains separate. The weaker parts are the ones that have undergone thermal
treatments (welding) and mechanical processes (moulding, bending, skirting, etc…) and that operate at
temperatures between 450 and 850° such as heat exchangers).

PITTING - Fig. C

Fig. C

This is the most common one on the
cooking appliances, very much on the
pans wells and brat-pans and easily
recognizable once the process is in
course due to the formation of black
points (from 0.1 to 1 mm) that can
even perforate the whole material
thickness. It shows when the stainless
steel is left in contact with the
contaminant solution (salt or water
having a very high percentage of
chloride) locally and in stagnant way.
The point shaped corrosive attack
operates in two phases, the first one
called “incubation phase” brakes the
passive layer, the second one called
“growing phase” presents a penetrating
or cavernous self-accelerating process
in the sense that feeds itself. From the
electrochemical point of view the
phenomena is caused by the formation
of a kind of battery between the metal
(cathode) and the contaminant (anode);
the electric current between the
cathode and the anode brings in the
solution (dilutes) the metallic ions so
creating the pitting. Chlorine ions CL­increase on the bottom of the pitting so
speeding the point penetration while
the lack of oxygen on the pitting
bottom prevents any attempt to

regenerate the passive film.

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