Corrosion and
corrosion-prevention methods.
by Grzegorz Olek
If we want successfully prevent the corrosion, we must now what
corrosion is and when its happened.
There is few types of corrosion, but most popular and arduousness are
electrochemical corrosion. Its happened everywhere where electrolyte
contacts with any metal conductor, like steal. Steal is melt of iron
and few more metals with carbon. One drop of distillated water is
enough for corrosion. Oxygen from air dissolve in water,
reduction-oxidation reaction begins to walk according to equation:
As we see, on left side we have 4 electrons. But from those electrons
gets? Answer is simply: From iron in steal. There is another
reduction-oxidation reaction:
Iron ions reacts with hydroxyl ions and we get hydroxyl of iron(II),
base component of rust.
How prevents this? Drop of water, steal and air creates miniature
electrical cell. If we want to stop corrosion process, we must stop
process of electrons transport from steel into electrolyte.
Methods:
1.
Using special, stainless steels. Method is good, if our part will
contacts with low-power electrolytes like sweet water or rain, part may
have large mass. Costs: 2-3 greatest then normal steels.
2. Using acid-resistant steels. Like in point one, but cost radically
increase: 5-6 then normal steels.
3. Protective coats of some metals (like copper, zinc, chrome), drifted
electrochemically. Method good if part is not in mechanism, and not
contacts with powerfully electrolytes, like ocean water. These coats
are not resistant on mechanical activity to.
4. Create another electrical cell and connect with protected part. This
method is so good and simple, that is used since a long time to protect
haul of sea-ships. Under water-line ship has blocks of zinc connected
with haul be wires. Corrosion attacks this blocks first, living steel
alone. Of course, this blocks needs to be supplied sometimes, otherwise
corrosion attack steel.
5. If we want resolve problem of corrosion in fixtures, we have no way.
Any pumps and valves parts must be made from acid-resistant steels,
pipes from copper or polymers, reactors from stainless or
acid-resistant steels (but some may be polymeric).
On Ocean Base we have another one problem with corrosion: salt. Ocean
water near equator has 34-36‰ of salt. Near equator we have sunny days
and high temperature. Water is still evaporating and salt crystallize
on every surfaces. Those crystals may strike root into structure of
steel. When that crystal broke out, another wave may wash out salt from
this point, creating ideal nest for electrochemical corrosion. Only
high temperature (110-130°C) may remove water from those nests.
Crystals of salt very easy creates microscopic holes in protective
metal coats. Water penetrates these holes and may create fireplaces of
corrosion (unless coat metal is zinc).
Gaseous hydrogen produced from water is to reason of corrosion.
Particles of gaseous hydrogen migrates into structure of all materials,
steel to. As I describe above, steel is melt contains carbon. This
carbon reacts with hydrogen into methane, which is imprisoned in steel
as bubbles. Those bubbles weaken steel structure. In 30-45°C this
reaction is very slow but after few years side of all tanks and
reactors are like strainer. If we want prevent this, all materials
contacted with hydrogen must be coated be some extremely expansive
compound of beryl (which is still in examinations in laboratories).
Other way is to replace all corroded parts (maintains cycle is almost
5-6 years for non-pressurized parts, and 2-2.5 years for pressurized).
In other materials hydrogen create microscopic pores, which weaken
structure of those materials. Maintains cycle for brass or copper is
almost 10 years, but those materials are more expansive and heavier
then steel. Copper is softer then steel, so copper sides must be
thickness then steel. Polymers are not good resolve, because those
react with hydrogen very easy.