Materials Sciences and Applications, 2012, 3, 587-595
http://dx.doi.org/10.4236/msa.2012.39084 Published Online September 2012 (http://www.SciRP.org/journal/msa)
587
Effect of Radial Expansion of Cr-Mo Steel Tubes on Their
Corrosion Behavior in Sea Water
A. C. Seibi
1*
, P. Rostron
2
, A. Elramady
1
, B. Mishra
3
, O. Al Nazer
1
, S. Al Ameri
1
1
Mechanical Engineering Department, Petroleum Institute, Abu Dhabi, UAE;
2
Chemistry Department, Petroleum Institute, Abu
Dhabi, UAE;
3
Metallurgical & Materials Engineering Department, Colorado School of Mines, Golden, USA.
Email:
*
aseibi@pi.ac.ae
Received March 22
nd
, 2012; revised May 7
th
, 2012; accepted June 19
th
, 2012
ABSTRACT
This paper studies the effect of expansion ratio on the corrosion rate of the expanded tubes exposed to sea water as well
as the microstructure changes caused by the expansion of Cr-Mo steel tubes to large permanent deformations. 2.25
Cr-1.0 Mo steel (schedule 40 and schedule 80) tubes were subjected to various expansion ratios of 10%, 15%, 20%, and
25% by pushing conical mandrels through them using a piston. The microstructure was also studied to determine the
effect of the expansion on the grain structure of the material. Microscopic examination of the expanded tubes revealed
that grains elongate along the direction of the hoop stress and this elongation increases with an increase of expansion
ratio. Moreover, it was found that corrosion rate and hardness increase with an increase in expansion ratio.
Keywords: Expanded Tubes; Corrosion in Seawater; Potentiostat; Hardness; Enlarged Grain Size
1. Introduction
Downhole tubular expansion of pipes (casings) is becom-
ing a normal practice in the petroleum industry to repair
damaged casings, shutdown perforations, and ultimately
achieve mono-diameter wells. Tubular expansion is a cold
drawing metal forming process, which consists of running
conical mandrels through casings either mechanically us-
ing a piston or hydraulically by applying a back pressure.
This mechanism subjects the pipes to large radial plastic
deformations up to 30% of the inner diameter. Extensive
literature related to the viability and advantages of this
technology in the field is summarized [1]. Experimental
and numerical studies were carried out in the past two
decades to understand the mechanical behavior of expanded
tubes/pipes. However, a limited number of research work
related to the corrosion behavior of plastically deformed
materials is available in the literature. The effect of the
degree of corrosion on the mechanical properties of steel
under plastic deformation was reported to be critical [2].
The study revealed that the hardness of 8 mm bars BSt500
s
steel reduced by 25% - 35% and 2% - 10% in the outer
and inner layers, which is due to removal of the marten-
site layer in the outer layer [3]. Moreover, it was found
that the increase in the degree of plastic deformation aids
the anodic and cathodic reactions between a steel boiler
and the acid solution that was used for flushing [4]. The
present paper, therefore, studies the effect of expansion
ratio on the corrosion rate of 2.25 Cr-1.0 Mo steel sam-
ples exposed to aerated sea water as well as the hardness
and microstructure changes of expanded tubes.
2. Theoretical Background
2.1. Stress Analysis
The mechanical response of tubes subjected to radial ex-
pansion was studied [5] by considering the equilibrium
conditions of an infinitesimal element (see Figure 1) from
which an expression for the hoop stress was deduced:
2
3
2
2
2 1 3
1
max
2 2
1
2 2
2 1 3
1 1
cos
cos
cos
h
m
r
h h h
r
q
t
r r
h h h
r r
(1)
where q
m
represents the contact pressure between the man-
drel and tube surface and is given by
2 2
2 1
(1 cot )
π (1 cot )
c m
m
F p
q
r r
(2)
The terms F
c
, p
m
, μ denote respectively the expansion
force, contact pressure, and friction coefficient. Equation
(1) expresses the stress in terms of the contact pressure,
*
Corresponding author.
Copyright © 2012 SciRes. MSA