Why are corrosion inhibitors needed during
hydrotesting
Oil tanks are huge in size ranging from 10 000 m
3
to over
70 000 m
3
, which is equivalent to about 600 000 bbls. For
their huge size, these tanks cannot be hydrotested with
treated water. Also, oil tanks are usually located near the
sea coast, hence seawater is easily available for carrying out
hydrotest.
While seawater is in abundance, it is highly corrosive. The
high salt content of seawater makes it rich in chloride, which
reacts with the metal and, in the presence of oxygen, converts
into hydrochloric acid and starts corroding the metal.
Prolonged storage in a metal tank causes aggressive
deterioration of the metal surface. Such corrosion can make
weak spots in the tank, which in the long coarse may lead to
tank breakage or leakage.
Corrosion related damage is accelerated by factors including
the tank’s interaction with interconnected components,
corrosive environmental conditions and stray electric currents.
Over time, uncontrolled corrosion can weaken or destroy
components of the tank system, resulting in holes or possible
structural failure, and the release of stored products into the
environment.
To prevent a corrosion attack on the inner metal surface of
the tanks, a result of all the salts present in the water, corrosion
inhibitors are recommended.
Types of corrosion encountered
The most common types of corrosion that are encountered
during hydrotest are:
)
Pitting corrosion – this causes pinhole damages to the metal.
The main cause of such corrosion is due to the formation of
hydrochloric acid.
)
Stress corrosion cracking – this leads to cracking of metal
due to high stress condition, which happens due to high
chloride content in water.
)
Oxidation corrosion – this type of corrosion causes uniform
rusting. This happens due to the high amount of dissolved
oxygen in the water, which causes oxidation of the metal
to form metal oxide i.e Fe oxidising to form FeO, Fe
2
O
3
or
Fe
3
O
4
.
)
Microbially induced corrosion – this type of corrosion
causes cavitation of the metal. This is due to high micro-
biological load present in water. The microbes cause
formation of acids and result in pitting of the metal.
Corrosion protection technique and chemistry
Modern corrosion control combines historically proven
methods with state-of-the-art technology to prevent tanks from
deteriorating. Corrosion-control strategies are used individually
or in combination with one another. A common mechanism for
the inhibition of corrosion involves the formation of a coating,
often a passivation layer, which helps to prevent the access of
the corrosive substance to the metal. Corrosion inhibitors serve
as additives to the fluids that surround the metal or related
object. Owners and operators of storage tanks must dedicate
the resources required to monitor and maintain these corrosion
protection systems to ensure the effective protection of the
environment and their economic interests.
Ideal corrosion inhibitor
A suitable corrosion inhibitor should have the following
properties:
)
Good oxygen scavenging properties.
)
Prevent formation of corrosive acids like hydrochloric and
hydrobromic.
)
Be in liquid form and be 100% and immediately miscible in
water.
)
Have a high pH.
)
Have a very low dosage.
)
Be effective enough to be used in open tanks or floating
roof tanks, where there is a greater chance of corrosion from
oxidation by air entrapment.
)
Organic and non-toxic.
)
Should not add COD and BOD.
)
Be easily dischargeable with no effect on marine life.
)
Eco-friendly.
)
Work in a wide range of temperature conditions.
)
Easy to store and apply.
There have been a lot of inhibitor compounds in the past,
like VCI, sulfites and such. A vapour corrosion inhibitor has a
lot of disadvantages, like having a restriction when it comes to
theoretical coverage, usually around the 15m
2
per litre mark.
Also, it takes a lot of time to actively work and needs to be
washed after use. A VCI also cannot be stored and used in
temperatures approaching zero (the recommended lower limit
is usually 2˚C, making it totally impractical and impossible for
storage in colder countries. While the conventional sulfite based
products work as cheap oxygen scavengers, they lead to high
sulfur content and the formation of sulfuric acid.
The most accepted chemistry for a hydrotesting corrosion
inhibitor is based on catalysed diethyl hydroxylamine or
catalysed DEHA. This compound helps to prevent corrosion
by forming a passive layer, working as an oxygen scavenger,
buffering pH and preventing formation of acid, and is non-toxic
to environment.
It meets most of the standards mentioned above and a
little customised tweaking by a good chemical manufacturer is
enough to accomplish a corrosion inhibitor that will meet all of
the standards.
Need of biocide
An organic corrosion inhibitor will inevitably invite bio-fouling
in the presence of sunlight. So, in case of an open-roof or a
floating-roof tank, dosage of a suitable low dosing biocide is
highly recommended. The amount of biocide in a corrosion
inhibitor is such that the components break down into non-
hazardous substances and there should be no residues. Even
78
World Pipelines
/
JUNE 2015
