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DEFINITION AND HISTORY
There are several ways for managing downhole corrosion,
from doing nothing to implement immune
materials like CRA. For the end user, the main purpose
of CRA tubing is not to resist corrosion but
to find out the overall less expensive solution. In
the oil and gas exploration, in many areas effluents
of the wells are corrosive and the trend is on the
increase.
The stainless steels appeared around 1870,
and at the beginning they were named “rustless”.
#01
CORROSION
RESISTANT ALLOY
for OCTG
At a glance the OCTG Corrosion Resistant Alloy (CRA) market is a
very small part: about 0.1% of the world wide stainless steel production.
But this part is fundamental for the exploration and production
of gas mainly in the more and more severe conditions.
COMPOSITION
Metallurgists consider steel as stainless if it contains
a minimum 10.5% Chromium that creates a
protective passive layer at the surface of the product
against corrosion. Some elements like Molybdenum,
Copper… can reinforce the corrosion resistance.
Stainless steels are sorted by ISO 13680 and API
5CRA in 4 classes related to their metallurgical
structure: martensitic, ferritic, austenitic-ferritic
(or duplex) and austenitic. Martensitic (mainly
13%Cr and super 13%Cr) are hardened by appropriate
heat treatment, but their use is limited to
Pilgering process Drawing process
.
The first target was to avoid corrosion in the guns
for weapons. Those rustless alloys then demonstrated
their advantages for many other usages like
in chemical industry. The true development of stainless
steels large mass production started in the
sixties with the steel melting and refining technologies
such as AOD and VOD. In the seventies the
development of gas research and production in cor
rosive wells leaded to develop specific OCTG
grades.
environment with low H2S partial pressures.
Contrary to duplex and austenitic, ferritic alloys
are not used in OCTG: their mechanical characteristics
are very difficult to strengthen by cold hardening.
Cold hardening (or also known as cold working or
strain hardening) is the strengthening of a metal
by plastic deformation. Shaping the product at
ambient temperature it allows reaching desired
yield strength and final dimensions. It is the main
process used for CRA.
T H E V A L L O U R E C O I L & G A S M A G A Z I N E # 0 1 - J A N U A R Y 2 0 1 2 13
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