Mechanical properties of CO 2 corrosion scale formed at different temperatures and their relationship to corrosion rate

Sample preparation

All specimens used in the present work were cut from a sheet of API P110 tubing carbon steel with a chemical composition of Fe–0·26C–0·19Si–1·37Mn–0·009P–0·004S–0·028Mo–0·028Ni–0·019Cu–0·011Ti (wt-%). The samples were machined with dimensions of 40×10×3 mm. Before each experiment, all the specimens were subsequently ground with 300, 600, 800 and 1200 grit silicon carbide paper and then polished with 3 and 1 μm alumina slurry to obtain a mirror-like surface. Final cleaning used distilled water and anhydrous ethanol; specimens were then weighed using a balance with a precision of ±0·1 mg.

A static autoclave (Cortest Co., USA) was utilised to produce corrosion scales at different temperatures. For each test, 12 samples were prepared in order to ensure the equivalence of corrosion scale growth. For each mechanical measurement (i.e. microhardness, elastic modulus, membrane–matrix bonding strength and fracture toughness), three replicate samples were used to obtain each of the specific properties the scale; another group of samples were used to determine the overall corrosion rate of the carbon steel. The test solution, simulating a formation water in an oil field, was made from deionised water (18 MΩ cm in resistivity) and analytical grade reagents with a concentration (g L⁻¹): 24·1 NaCl, 37·4 KCl, 0·7 CaCl₂, 2·6 Na₂SO₄ and 0·6 MgCl₂.6H₂O. Before tests, pure N₂ was aerated into the solution and bubbled for 12 h in order to remove dissolved oxygen. The autoclave temperature was controlled to the set values of 50, 70, 100 and 130°C, the CO₂ partial pressure was maintained at 4 MPa and the total pressure was 10 MPa, the exposure test was carried out for 168 h. After testing, the corroded samples were rinsed with deionised water, dried with anhydrous ethanol and divided into two groups: one group was kept in desiccators for mechanical testing and the other group was used to analyse scale morphology and calculate corrosion rate.

Mechanical properties test

After any loose scale on the surface was removed by light abrasion, the hardness of scale was measured using a nanometer mechanics probe (Nano indenter II) produced by the MTS company with a load of 10 mN; the elastic modulus of CO₂ corrosion scale can be obtained based on the load–displacement curve. The fracture toughness K _IC was measured on polished cross-sections of corrosion scale by means of Vickers’ microhardness indentation tests, which was calculated according to equation (1) 9 9,10 (1) where P is the applied load (N), E is the elastic modulus (GPa), H is the hardness (GPa), 2c is the total length of the crack (m) including the diagonal length of the indent d (m) and coefficient α = 0·04.9 By substituting H = 2Psin68°/d2 (Ref. 11) into equation (1), equation (2) can be obtained (2) A tensile testing machine with a loading speed of 0·5 mm min⁻¹ was utilised to obtain the bonding strength of corrosion scale and epoxy resin structural adhesive was used to ensure the opposing testpieces together. The schematic diagram of lap shear and tensile pull-off measurements is shown in Fig. 1.

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Figure 1

Schematic diagram of bonding strength test

Corrosion rate and morphology

The products on the corroded samples were removed based on the chemical cleanup method according to GB/T 16545-1996 (idt ISO 8407:1991), the film removing liquid is composed of 1 L HCl (ρ = 1·19 g mL⁻¹), 20 g Sb₂O₃ and 50 g SnCl₂, in which the corroded samples were immersed for 10 min, then rinsed with deionised water and anhydrous ethanol, dried in air, and weighed again to calculate the corrosion rate. The corrosion rate C _R was reported in mm/year according to the mass loss method via equation (3) (3) where W ₀ and W ₁ are the original weight and final weight of specimens (g) respectively; S is the exposed surface area of specimens (mm²); t represents the immersion time (h); and ρ is the steel density (7·8×10⁻³ g mm⁻³ is used in the present work). Surface and cross-section morphologies after corrosion test, and the surface morphology after adhesion testing, were characterised using scanning electron microscopy.

Hardness and elastic modulus

Figure 2 shows the load–displacement curve of CO₂ corrosion scale formed at 100°C, based on which the elastic modulus of the corrosion scale was obtained. The hardness and elastic modulus of the scales formed on P110 carbon steel at different temperatures are shown in Fig. 3, showing an initial decrease followed by an increase with increasing film formation temperature, and where the minimum values were found at 100°C. Generally speaking, the hardness depends on the structure of the CO₂ corrosion scale such that the looser the structure, the lower the hardness. The elastic modulus is concerned both with the atomic structure of the scale, which is mainly composed of FeCO₃, 3 3,4 and with its porosity. The elastic modulus was found to decrease with increasing the porosity of corrosion scale: the elastic modulus of single crystal FeCO₃ was 185 GPa, which decreased to 80 GPa when the porosity of CO₂ corrosion scale was 50%.12 In the present paper, the elastic modulus of the scale was found to lie in the range 101-160 GPa, which indicated that there was significant microporosity in the scale and was the reason why the matrix still suffered from corrosion though the protective product film (FeCO₃) covered on steel surface.

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Figure 2

Load–displacement curve of CO₂ corrosion scale formed at 100°C by nanohardness tester

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Figure 3

Elasticity modulus and hardness of CO₂ corrosion scale formed at different temperatures

Fracture toughness

In order to avoid the roughness of the corrosion scale affecting the test results, the fracture toughness K _IC was measured by means of Vicker's indentation tests on polished scale cross-sections and calculated based on equation (2). The fracture toughness values decreased to a minimum value and then increased with increasing the film formation temperature as shown in Fig. 4. Yu et al.7 thought that fracture toughness might depend on the grain size of the film, i.e. a bigger grain size could result in a decrease in fracture toughness. At the same time, the thickness of the film also affects fracture toughness; because CO₂ corrosion scale is brittle, it is easier to initiate a crack on the top of indentation. A thicker scale would limit the growth of the perpendicular crack and then might promote the propagation of a horizontal crack. Yamashita et al.13 found that CO₂ corrosion scales formed at 100°C were thicker and looser. So the fracture toughness of corrosion scale formed at 100°C was the least.

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Figure 4

Fracture toughness of CO₂ corrosion scale formed at different temperatures

Bonding strength

The bonding strength between the corrosion scale and the matrix is shown in Fig. 5 and presented a similar trend to the other mechanical properties mentioned above. The morphology of the scale after the bonding strength test is shown in Fig. 6. When the film suffered from shear, the fracture surface of the film was relatively orderly showing brittle cleavage fracture as shown in Fig. 6a. While there were some cavities evident on the surface of the film as shown in Fig. 6b after testing in shear, this might be due to an uneven interface. However, under a tensile force, it was easier for corrosion products to be taken pulled out from low pits, resulting in the formation of cavities.

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Figure 5

Bonding strength of CO₂ corrosion scale formed at different temperatures

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Figure 6

Morphology of CO₂ corrosion scale formed at 100°C after bonding strength testing

Corrosion rate and morphology

The corrosion rate of P110 tubing steel increased to a maximum value and then decreased with increasing the temperature. The maximum value lay at 100°C as shown in Fig. 7, which is in line with previous research,14 and the variation in corrosion rate is inverse to that of the mechanical properties of corrosion scale with increasing the temperature. Figure 8a shows the morphology of CO₂ corrosion scale formed at 100°C; the structure was looser and more porous than that formed at other temperatures. The cross-section micrographs of corrosion scale formed at 100°C as shown in Fig. 8b also showed this effect with the scale being thicker (∼200 μm) and composed of two layers, with the outer layer covered by looser deposits with a thickness of 40 μm. This structural characteristic of the corrosion scale determined the variation trend of mechanical properties. It is interesting that P110 tubing steel suffered from uniform corrosion based on the even interface between corrosion scale and the matrix.

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Figure 7

Corrosion rate of P110 tubing steel at different temperatures

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Figure 8

Morphology of CO₂ corrosion scale formed at 100°C

Relationship of mechanical properties and corrosion rate

As is well known, when a material is covered with a protective corrosion scale, generally the more compact the structure of the scale the smaller the corrosion rate. The hardness of CO₂ corrosion scale shows the resistance to deformation or fracture caused by the extraneous pressure and reflects the cohesive bonding strength among atoms or molecules in the film layer. The adhesive bonding strength between film and matrix refers to the blocking property at the interface and also reflects the bonding strength between atoms or molecules of the film layer and iron atoms of the matrix. Since tube steel suffers from mechanical influences under the operating condition, such as the shear stress of different fluid states, load and the internal stress, etc., the mechanical properties of corrosion scale determines the extent of damage and reflects the protective degree for the substrate. Thus, reduced mechanical properties of the CO₂ corrosion scale can more easily lead to cohesive (internal) scale rupture and also adhesive failure away from the matrix. In this condition, a greater superficial area of the matrix will be exposed to the corrosive medium, which promotes the increase in corrosion rate. The relationship between corrosion rate of P110 carbon and hardness of corrosion scale formed at different temperatures as shown in Fig. 9.

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Figure 9

Relationship between corrosion rate of P110 carbon steel and hardness of corrosion scale formed at different temperatures