Laboratory test practice standards for corrosion measurement such as NACE and
ASTM try to obtain accurate reproducibility of the results. To study effects
of surface condition, it can be, generally, conducted by removing a substantial
layer of metal from the test specimens. This method should be done to eliminate
effect of metallic surface variations. However, the lack of specific guidelines
leads to variations in surface roughness which can affect the corrosion rate
measurement. According to NACE (NACE Standard TM0169-2000,
2000), a common and widely used surface finish is produced by polishing
with No. 120 abrasive paper or its equivalent. Ideally, in laboratory test,
the surface of the specimen should be identical with the surface of actual equipment
to be investigated. However, this is usually not possible as the surfaces of
field equipment vary as fabricated and due to its interaction nature with environment.
Since many interactions factors may govern corrosion behaviour, sometimes, there
is a different result between experimental data and field data. One of the factors
that might cause those differences is surface conditions. It was described theoretically
that surface roughness can influence electrochemical or mechano-chemical behavior
of a surface. Such conditions could promote different interaction in the micro-electrical
behavior and will impact on corrosion rate. Thus, effect of surface roughness
is important factor that should be considered in verifying corrosion rate of
carbon steel from the laboratory experiments and corrosion rate of as-delivered
internal pipe line. This study investigated the correlation between various
surface roughness conditions on corrosion rate in CO2 environment.
Literature review of surface roughness: A number of studies have been
carried out to investigate various aspects of surface roughness in relation
to the corrosion rate. Cheng and Roscoe (2005) investigated
the influence of surface polishing on the electrochemical behavior of titanium.
The research concluded at high anodic potential range (>2.0V), the 1 μm
diamond paste polished electrode gave a much higher anodic current than the
rough sandpaper polished. Klein et al. (2005) investigated
the erosion/corrosion resistance of chromium nickel steel in the as-delivered
condition and the surface roughness of the heat treated version in a multiphase
CO2 corrosion. The results indicated that one of the heat treated
versions of the steel has higher wear resistance.
According to Fogg and Morse (2005), steel pipedelivered
to the coating yard has a relative roughness in order of 20 μm and may
exceed 50 μm, depending on the corrosion products formed on the surface.
This, in turn, affects the fluids in motion. Friction occurs between the fluid
and the pipeline wall will affect on corrosion rate.
Fogg and Morse (2005) have also done a research in 2005
on the effect of surface roughness on the maximum flow rate of a subsea gas
export line. They concluded that the maximum flow rate decreases with increasing
internal surface roughness. It was due to turbulent flow of natural gas transportation
in pipelines can form a laminar film at the pipe wall which reduce the friction
between the fluid and pipe wall. Increased roughness would also affect the hydrodynamic
boundary layer, mass-transfer boundary layer, thus affecting the fluid-velocity
sensitive corrosion mechanism.
CO2 corrosion: Corrosion in oil and gas industries mainly
deals with CO2 gas as it is the main species presenting in oil field.
The hydration of CO2 to carbonate acid causes corrosion on mild steel.
Carbonate acid decreases pH of the medium. Degree of corrosiveness due to CO2
gas is influenced by environmental conditions such as; temperature, CO2
partial pressure, corrosion film properties and flow conditions (De
Waard and Milliams, 1975; Silverman, 2006). In aqueous
environment, CO2 corrodes carbon steel through an electrochemical
process involving anodic dissolution of iron and cathodic evolution of hydrogen
(Nesic, 2007; Nordsveen et al.,
2003). When water reacts with CO2 gas, it will produce carbonic
acid as shown in the following reaction equation:
On the anodic site, the anodic dissolution of iron in acid solutions occurring
as follows (Nesic, 2007):
Then, on the cathodic side, there are two possibilities reactions are independent
and the net cathodic current is the sum of the currents for the two reactions:
The overall reaction is:
From the reaction in Eq. 2, anodic process of iron oxidation
could produce FeCO3 and/or Fe3O4 film which
can be protective or non protective depending on the conditions when it forms
(Schmitt and Papenfuss, 1999).
MATERIALS AND METHODS
Electrochemical set-up: A typical schematic three-electrode set-up used
in all electrochemical experiments is presented in Fig. 1.
A rotating cylinder electrode with a speed control unit was used as the working
electrode. Glass cell was fitted with graphite electrodes as auxiliary electrode
and a Ag/AgCl as a reference electrode.
Specimen preparation: The working electrodes were carbon steel and the chemical composition is as shown in Table 1. The cylindrical specimens have diameter of 12 and 10 mm length. Before immersion, the specimen surfaces were polished successively with 60, 240, 400, 600, 800 and 1200 grit SiC paper, rinsed with methanol and decreased using acetone. The experiments were repeated at least twice in order to ensure reasonable reproducibility.
Cell solutions: The experiments were performed both in stagnant and flow solutions condition. The total pressure was 1 bar, the glass cell was filled with 1 L of distilled water and 3% wt. NaCl which was stirred with magnetic stirrer. Then, CO2 gas was bubbled through the cell (at least one h prior to experiments) in order to saturate and de-aerates the solution. Temperature was set using a hot plate. After the solution was prepared, the pH was adjusted to reach the pH set by using NaHCO3 as buffer solutions.
||Schematic RCE corrosion test cell (George,
2007); 1: Reference electrode 2: Gas in; 3: Gas out; 4: Luggin capillary;
5: Counter electrode; 6: Rotator; 7: Temperature probe; 8: pH probe; 9:
||Composition of 080a15 (Bs 970) carbon steel used in the experiments
||Surface profiles for different surface finishes
|| Experimental test matrix
Simulation of flow condition was conducted using Rotating Cylinder Electrode
(RCE). A cylindrical working electrode was screwed onto an electrode holder
at the center of the cell for rotating in the RCE. The Linear Polarization Resistance
(LPR) technique was used to measure the corrosion rate. The procedure is similar
to ASTM Experimental test G 5-94 (ASTM G 5-94, 2004).
The Mitutoyo Surface Profiler was used to measure the roughness as Ra and Rz parameter, the arithmetic average and depth of the peak-to-valley height of surface asperities in micrometers (μm). An average of six random readings is taken, as shown in the subsequent Table 2. More detail results regarding to the surface condition and SEM investigation can be seen in Appendix.
Test matrix: The corrosion evaluations in this study will be performed under stagnant and dynamic conditions, with the use of static electrodes and RCE apparatus. Table 3 was test matrix used to do experiments.
RESULTS AND DISCUSSION
Corrosion rate in static condition: Effects of surface roughness on
corrosion in CO2 saturated solution is presented in Fig.
2. The figure shows that increase of corrosion rate as a decrease surface
roughness. Figure 2 was also seen that material with rough
surface will prone to be stable compared to smooth surface during 2 h measurement.
At 1200 grid surface finishing, corrosion rate fluctuate from 1.5-2.5 mm year-1.
But, variation of corrosion rate occur around 1 mm year-1 for material
with 60 grid surface finishing. The effects of surface roughness on contributing
an increase of corrosion rate were also investigated by (Li
and Li, 2006).
Measurement corrosion rate with different corrosion measurements method is
presented in Fig. 3. It can also be shown that generally the
corrosion rate of the rough surface is higher than the smooth surface finish.
Rough surface increases corrosion rate by increasing surface area which involves
in distribution of electrochemical reaction.
||Average corrosion rate of carbon steel in CO2-saturated
NaCI solution at pH 5.5,22°C, static condition at several surface roughness
||Corrosion rate (mm year-1) for variation surface
finishes at static condition as measured using different corrosion measurements
method (EIS, LPR, and weight loss)
Corrosion measurement using EIS and weight loss showed a consistence decrease
of corrosion rate for rough surface and smooth surface. But, LPR presented fluctuating
Corrosion rate in flow conditions: From Fig. 4, there is a trend that corrosion rate is higher at rougher surfaces. It can be concluded that a rise in surface roughness intensifies corrosion processes by increasing the surface contact of the specimen subjected to corrosion, as can be seen in the Rz surface profile (Appendix 1). These effects may relate with increasing distribution of fluid phase, mass transport species and giving mechanical forces that could wash away corrosion products.
Figure 5 is presented comparison different corrosion methods in measuring corrosion rate on flow condition. Results from all measurement methods suggested that the corrosion rate calculated by EIS and LPR did not vary significantly in turbulence flow condition. There was also indication that the corrosion rate did not much different along the surface roughness tested. However, using LPR method, the corrosion rate showed a meaningful decreased.
||Average corrosion rate of mild steel in CO2-saturated
NaCl solution at pH 5.5, 22°C, 100° rpm several surface roughness
||Corrosion rate for variation surface finishes at flow simulated
condition (1000 rpm) as measured using different corrosion measurements
method (EIS, LPR and weight loss)
||Based on surface profiles and corrosion rate measured, the practice of
400, 600 and 800 grit surface finish is acceptable in representing the as-delivered
||As the corrosion rate varies at different roughness, the abrasive paper
used for surface polishing must be consistent throughout the experiment.
Too rough or fine finish should be avoided as it may induce high inaccuracy
in predicting field corrosion behavior
||Rough surface, as shown by the surface profile, has larger area of surface
contact with the corrosion environment. Thus, a rise in surface roughness
intensifies corrosion processes by increasing the surface contact of the
specimen subjected to corrosion
||It can also be concluded that generally, the corrosion rate of the turbulent
flow is higher than static condition for all surface finish.
||The corrosion rate difference between surface finishes at static condition
is relatively small and insignificant
The results leave a gap between the corrosion rate measured by EIS with weight loss and LPR. Since the EIS results show significant corrosion rate deviation in turbulent flow, it can be further investigated in the future. This study can also be further improved by conducting experiments at higher temperatures and by including the effect of film formation at the surface.
Financial support from Universiti Teknologi PETRONAS (UTP) to undertake the research work is highly appreciated.
||Surface Profiling and SEM Imaging (500x Magnification); (a)60
Grit, (b) 400 Grit and (c) 1200 Grit