An estimate is given of the effect of strain ageing on the corrosion resistance of pipe steel 17GS in a

corrosive operating medium. Metal structure and mechanical properties after strain ageing are considered

in two conditions, i.e., after controlled rolling and after normalizing. It is shown that “aged” steel after

controlled rolling exhibits greater corrosion resistance than normalized steel.

As is well known [1], the majority of pipe steels of grades for oil and gas are subject during operation to strain age-

ing, which is reflected, on the one hand, by an increase in strength properties, and, on the other hand, by a marked reduction

in steel resistance to failure (embrittlement). The main strain ageing mechanisms are known, whereas the connection of this

phenomenon with metal corrosion resistance has not been studied sufficiently.

The aim of this work is to evaluate the effect of strain ageing on corrosion resistance of pipe steel in an operating

medium.

Tests were performed on pipe steel 17GS (composition, %: 0.18 C, 0.41 Si, 1.2 Mn, 0.013 P, 0.016 S, 0.025 Cr,

0.080 Ni, 0.013 Cu). The steel was studied in two original conditions: after controlled rolling and after normalizing. Metal

ageing was carried out by strain in tension (10%) followed by tempering with T = 250°C for 1 h and air cooling (GOST 7268).

Specimens of “aged” steel had a prismatic shape with a size of 12 × 12 × 60 mm with a working face in the form of

a microsection (11 × 12 mm) in contact with the corrosive medium.

Before the start of tests, the working face was degreased and subjected to acid etching in 5M HCl in order to remove sur-

face damage after machining. The nonworking surface of specimens was coated with chemically resistant insulating material.

The corrosive agent used was 30% NaCl solution in distilled water with pH = 4.

Experiments were performed in two stages: corrosion tests; coulometric recording of the corrosion products. During

corrosion tests in a corrosive medium (volume V = 60 ml; stirring rate v = 20–25 cm/sec), the working face of a specimen was

immersed and held successively for 0.5, 1, 1.5, 2, 3, and 4 h. A “one-time” specimen was used for each time point.

The corrosion products formed at the working face during tests were removed in a solution of specially remover with

pH = 3.8. In parallel, in order to determine background corrosion indices in the removal solution, a control “aged” specimen

was exposed to it for 1 h without mixing. The solutions obtained for the test medium and remover with “aged” specimen cor-

rosion products were stored in 0.1M HCl, after which coulometric analysis was carried out.

Chemical and Petroleum Engineering, Vol. 44, Nos. 3–4, 2008

STRAIN AGEING AND CORROSION RESISTANCE

OF PIPE STEEL

A. E. Kuzmak,1 A. V. Kozheurov,1

L. A. Efimenko,2 V. Yu. Ilyukhin,2

and O. V. Konovalova2

MATERIALS SCIENCE AND CORROSION PROTECTION

Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 3, pp. 47–48, March 2008.

0009-2355/08/0304-0177 ©2008 Springer Science+Business Media, Inc. 177

1 Frumkin Institute of Physical Chemistry and Electrochemistry (IFKhÉ), Russian Academy of Sciences, Moscow.2 Gubkin Russian State University of Oil and Gas, Moscow.

178

Fig. 1. Microstructure of steel 17GA in the original condition (×200): a) after controlled rolling;

b) after normalizing.

Fig. 2. Kinetics of the overall amount of corrosion products, i.e., Fe3+ + Fe2+ (a) and Fe2+ ions (b)

of steel in the original condition: 1) after controlled rolling; 2) after normalizing.

Fig. 3. Kinetics of the overall amount of corrosion products, i.e., Fe3+ + Fe2+ (a) and Fe2+ ions (b)

of steel after strain ageing: 1, 2) same as in Fig. 2.

Corrosion products (Fe3+ and Fe2+ ions) were recorded by direct potentiostatic coulometry (DPC) [2, 3]. The prima-

ry measuring element used was an indicator electrode-cell made of glass-carbon SU-2000, the auxiliary electrode is carbon

fiber, and the reference electrode is KhSÉ.

The steel has a ferrite-pearlite structure. Within the metal after controlled rolling (Fig. 1a) the pearlite has a lined

distribution and a lamellar structure. The content of pearlite in the structure is 40–35%. Metal hardness is 2000 MPa.

The differing grain size of ferrite (60–65%, grain size 8–30 µm) should be noted, and this is explained by formation

during controlled rolling of ferrite phase nuclei both at the boundaries and within grains of the original austenite. The struc-

ture is in a stressed state.

As a result of normalizing (Fig. 1b), the banded structure is disturbed. The content of pearlite in the structure is about

50%, and it retains a lamellar form of precipitate. The ferrite component is grains with a size of 0.01 mm. the hardness of the

structure is 1930 MPa. Ferrite and pearlite are uniformly distributed throughout the volume of the metal.

Results of coulometric measurements indicate that a difference in structures governs the different corrosion behav-

ior. It follows from Fig. 2a that the overall weight loss ∆m (sum of Fe3+ + Fe2+ ions in the reaction volume, which is a part

of the space of the interface where the metal is in contact with electrolyte) is higher for steel after controlled rolling than for

normalized steel.

In this case, the emergence of Fe2+ in the surface area of the metal (corrosion rate) of steel after rolling is somewhat

higher than for normalized steel (Fig. 2b). Approach of the corrosion curves, starting with the second hour of testing, is due

to a reduction in the amount of Fe2+ ions due to the transition Fe2+ → Fe3+ under the action of oxygen that is present.

As a result of strain ageing, there is a marked change in all the mechanical properties of steel 17GS (see Table 1).

The increase in steel ultimate strength after controlled rolling is about 17%, and for the yield point it is about 57%.

For steel in the original normalized condition, the increase in ultimate strength is about 19%, and for the yield point it is more

than 76%. For both original steel conditions, there is a marked reduction in metal ductility characteristics. As can be seen, in

the case in question the greatest strengthening occurs for steel in the normalized condition.

Strengthening after strain ageing leads to a reduction in overall corrosion (sum of Fe3+ + Fe2+ ions) for controlled

rolled steel compared with normalized steel (Fig. 3a).

This result may be explained by metal strengthening and a corresponding reduction in the heterogeneous component

of the steel surface after controlled rolling in contact with electrolyte in the deformation zone.

An indicator of the increase in corrosion resistance for steel after controlled rolling is the kinetics of Fe2+ ion for-

mation within the reaction volume (Fig. 3b).

It may be concluded from the results obtained that a measured approach based on using the DPC method provides

reliable recording of the corrosion process.

Thus, it has been established that during testing (5 h) the overall weight loss (sum of Fe3+ + Fe2+ ions) at the metal

surface and in the corrosion medium is higher after controlled rolling than for a normalized structure.

It has been demonstrated that “aged” steel after controlled rolling exhibits a greater corrosion resistance than nor-

malized steel. The corrosion resistance of steel may be specified by the kinetics of the amount of Fe2+ ions.

179

TABLE 1

Level Hardness a

Ultimate Yield point

Relative Relative Metal strengthe-

Original condition of strain HV, MP

strength σ0.2, MPa

elongation reduction of ning factor

ε, % σu, MPa δ5, % area ψ, % Ky1 Ky2

After controlled rolling 0 2000 587–603 425–440 25–31 67 – –

10 2220 687–706 668–692 8.8–15 62 1.17 1.57

After normalizing 0 1930 606–616 406–414 27–31 68 – –

10 2430 723–743 715–736 15.2–17.6 60–64 1.2 1.8

REFERENCES

1. A. M. Zinevich, “Provision of reliability for main pipeline functioning,” Zashch. Metallov., No. 11, 57 (1992).

2. A. E. Kuzmak and A. V. Kozheurov, “Coulometric evaluation of carbon steel corrosion rate,” Zashch. Metallov., 4,

105–109 (2004).

3. A. E. Kuzmak and A. V. Kozheurov, “Coulometric evaluation of carbon steel corrosion rate,” Korroziya: Mater.,

Zashchita, No. 1, 81–85 (2005).

4. O. I. Steklov, Welding with Controlled Heating Cycles for Oil and Gas Industry Structures [in Russian], MINKh and

GP, Moscow (1985).

5. A. P. Gulyaev, Metallurgy [in Russian], Mashinostroenie, Moscow (1986).

6. N. P. Lyakishev, M. M. Kantor, and V. N. Voronin, “Study of the structure of gas conduit metal after prolonged oper-

ation,” Metally, No. 1, 15–24 (2005).

180