WHY TAKING CARE TO

PREVENT CORROSION

Some rules on how avoiding and solving these phenomena

Metal corrosion is too often underestimated though it is someti-

mes really damaging to heat systems.

It is a chemical and electrochemical phenomenon which can

occur whenever one or more metals are in a wet environment or

in contact with a liquid (as in the case of the pipes of a system

or of a heat exchanger) or even in a dry environment.

HOW DOES IT OCCUR: First of all we must consider the

tendency of some metals to oxidize more easily than others

as specified by the classification hereunder defining the oxida-

tion-reduction potential according to which higher and positive

values indicate a higher stability of metal to the detriment of

other metals having lower potential values.

Inclination to oxidation of some metals:

COPPER > NICKEL > TIN > LEAD > CHROMIUM

(nobler metals)

IRON > ALUMINIUM > CADMIUM > ZINC > MAGNESIUM

(less noble metals)

Let’s take the example of Iron (one of most easily oxidized

metals). If Fe is placed in a wet environment or in contact with

water, as in a real pile, it turns into Fe++ +2 e- (i.e. each iron

atom frees two electrons). Electrons “run” through the metal

attracted by the positive pole (cathode) and combining with

hydrogen positive ions in water they produce gaseous hydro-

gen H

2

(flammable hence dangerous). Another consequence of

these simple chemical reactions occurring in the negative zones

of metal called “anode” is the dissolution of metal Iron which

turns into Fe (OH)

2

ferrous hydroxide, an unstable compound

depositing in the anodic area and subsequently turning into

oxide. The cathodic zone is instead protected. This happens in

theory, because actually in the case of Iron, corrosion is “auto-

catalytic” often proceeding with irremediable consequences.

Other metals such as Aluminium (within a pH range 4 to 8.5) or

Copper form a thin film of “stable” surface oxides protecting the

metal surface against advancing corrosion. Outside the range

above mentioned and if not properly protected, Aluminium

corrodes as well as Stainless steel iron; in fact the latter is not

actually stainless but, since it consists of an alloy of iron with

nobler metals such as Chromium and Nickel, iron is protected

by an imperceptible film of “stable” surface oxides formed by

Chromium and Nickel. Being a water system made from dif-

ferent metals (hence having different degrees of nobility) the

weakest metal “sacrifices” oxidizing to the advantage of nobler

ones. This nearly always happens to iron, but be also careful to

aluminium.

MAIN CAUSES:

- Presence of different types of metals (pile effect)

- Stray currents (earthed)

- Little non-uniformities of metals

- Difference in temperature and internal stress

- Different concentrations of salts and gases inside water.

Sometimes corrosion can stop or slow down very much since

the difference in potential between the cathodic and the anodic

zones decreases with the passing of time. Other causes could

make it proceed:

- Acid pH (lower than 7)

- The presence of oxygen dissolved in water, i.e. “open” circuits,

such as those continuously fed, for example steam boilers where

phenomena are exasperated or open cup systems or radiant flo-

ors (insufficient oxygen barrier), are more subjected to corrosion.

- The presence of carbon dioxide which, dissolved into water,

forms carbonic acid lowering pH.

- Temperature

When not protected iron oxidizes in different way like Fe(OH)

2

ferrous hydroxide, Fe(OH)3 ferric hydroxide, Fe

2

O

3

oxide hydrate

or rust, Fe

3

O

4

magnetite, FeO ferrous oxide. On the contrary iron

would find its stable equilibrium in a clearly alkaline environment,

i.e. with a pH 9.7. Nevertheless, according to “Nerst’s theory” the

deposit of Fe(OH)

2

produced by corrosion would not resist to

the high speed of water flow in pipes. Other phenomena are in

“favour” of who use iron exchangers; in fact, within a temperature

range from 200 °C to 570 °C, a protective and magnetic oxide

strictly adhering to the iron surface like magnetite, Fe

3

O

4

appears.

However, when temperatures above mentioned are exceeded,

magnetite decomposes in ferrous oxide (FeO) of powdery nature

which often clogs exchangers and the pump elements.

The main causes of magnetite dissolution are:

- Film boiling which becomes manifest at high temperatures &

pressures and causes local wall overheating;

- Flame impingement, instability of flames which can cause local

overheating of pipes with the consequent detachment of magne-

tite flakes.

- Local thickening of the oxide film, contributes to increase the

metal surface temperature.

- Local deposition of limestone causing a non-uniform diffusion

of heat.

Other phenomena can worsen the state of systems, in particular

DEPOSITION PHENOMENA (SLUDGE AND MUD): Sludge and

mud, are often present and cause malfunction. Independently of

their nature, our prevention program represents one of the main

objectives of our treatments.

A “filthy system” jeopardizes not only the efficiency of the heat

exchange, but also the effectiveness of anti-corrosive treatment.

In fact, if a surface is clean, i.e. free of any deposits, any possible

corrosion phenomena will be of a uniform type and will therefore

be less dangerous (if compared to the effects caused by localized

corrosion); deposition phenomena can be identified as follows:

- Incrustations

- Fouling

- Biofouling

Incrustations occur for the crystal growth of an adhering layer

of insoluble salt (limestone) or oxide (silica) on surfaces where

the heat exchange occurs. Water hardness can to a certain

extent contribute to protect metals against corrosion, although

the opposite problem causing limestone deposition shall not be

incurred, too.

Fouling is better known as the deposition of substances such

as sludge or iron suspended in make-up water or of organic

substances naturally present, particles introduced by the atmo-

sphere, slash disposals such as: hemp, low quality sealing paste,

gaskets, cutting oils, incoherent oxides formed in the yard due to

the prolonged stops, leaks of softening resins, etc.

Biofouling. Microorganism causing biofouling can penetrate the

system through different ways. They can be present in make-up

water or in air (insufficient oxygen barrier). Other bacterial species

such as spores, iron bacteria, etc. besides moulds and yeasts (the

latter remarkably complicate things since they produce abundant

amounts of silt).

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