Protection enhancing of threaded connections of light-alloy drill pipes against contact corrosion

. When using light-alloy drill pipes (LAIDP) with steel tool joints, the development of contact corrosion is observed under certain operating conditions. The value of corrosion mainly depends on the difference in electrochemical potential (ECP) of the contacting metals. One of the effective methods for increasing the corrosion resistance of aluminum alloys is the micro-arc oxidation (MAO) method. This is an electrochemical process in combination with micro-arc-discharges phenomena at the anode-electrolyte border, which allows forming ceramic coatings of aluminum oxides on the surface, including its high-toughness and wear-resistant phase - α -Al 2 O 3 (corundum). MAO-technology is a highly efficient and environmentally friendly process. At the forming of such a coating on the threaded part and in the tool joint zone of the pipe, a barrier for contact corrosion between the steel tool joint and the surface of the aluminum pipe is created. In this work, contact corrosion on samples in a pair of 1953T1 aluminum alloy 40KhN2MA steel in a 5% NaCl solution at 80 °C was investigated. The data obtained showed the effectiveness of using protective MAO-coating to reduce contact corrosion and increase the reliability of the tool joint threaded connection of LAIDP.


Introduction
The increase of the share of horizontal wells, wells with a complex profile, and multilateral wells is a pronounced trend in the development of drilling for oil and gas in the last decade [1]. When drilling such wells, preconditions are arisen for the use of LAIDP-type drill pipes (lightalloy improved dependability drill pipes). Their design is a pressed pipe made of aluminum alloy (for instance, 1953T1) with upset ends, on which a trapezoidal thread with a stabilizing shoulder is cut for attaching the box and the pin of a drilling tool joint made of steel 40KhN2MA (Fig. 1). The use of LAIDP allows us to reduce the loads during rotation and movement of the drill string and ensure the achievement of farther extended reach drilling targets, in comparison with steel strings, if all other conditions being equal [2]. The difference in electrochemical potentials of the contacting materials in LAIDP with a steel tool joint in a corrosive environment is often a source of contact corrosion, which is reflected with a greater extent in the aluminum pipe section adjacent to the box [3][4][5]. Such a corrosion damages occurred, for example, when drilling at the Tarim oil field in the People's Republic of China [6].
To reduce or eliminate contact corrosion in the aluminum-steel connection, it is necessary to create conditions that provide either displacement of the electrode potential or the absence of direct metal contact by applying, for example, an insulating coating. The latter approach was investigated in work [7], in which an anodic oxide film was used as such a coating, and showed the effectiveness of its application.
In the processing of aluminum alloys to increase their corrosion resistance, wear resistance, and other surface properties, recently, along with traditional anodizing, the micro-arc oxidation (MAO) method is widely used.
The MAO method [8][9][10][11] is an electrochemical process of ceramic coating formation under the action of micro-arc discharges on the alloy surface, consisting mainly from oxides of the processed aluminum alloy including its high-strength phase alpha-Al2O3 and compounds of an electrolyte.
MAO technology is a highly efficient and environmentally friendly process. The forming of such a coating in the joint-pipe zone and on the threaded part of the pipe creates a barrier for contact corrosion between the steel joint and the surface of the aluminum pipe. In this case, the density of corrosion currents decreases due to an increase of electrical resistance of the contact and the creation of an electrical insulation barrier in the part of the pipe adjacent to the joint, which is a potential zone of corrosion damage.
The use of MAO-coating in the manufacture of LAIDP pipes was recommended in 2013 when the experimental processing of the thread with a stabilizing shoulder on the pipe was performed [3]. Then the first studies were carried out on samples simulating the connection zone [4]. The results were promising, but research has not been continued.
This paper presents the first stage of a new research program on the possibility of increasing the corrosion resistance of pipes made of 1953T1 aluminum alloy against contact corrosion by forming the MAO-coating in the contact zone of the pipe.

Materials and investigation methods
The samples for contact corrosion tests simulating a threaded connection "steel tool joint -aluminum pipe" were made from aluminum alloy 1953T1 with dimensions: 100 mm long, 10 mm in diameter and a threaded part (М8-6g) with a length of 17 mm, the couplings were made of steel 40KhN2МА with dimensions: length -25 mm, diameter -15 mm, thread -М8-6Н.
Before testing, the samples were thoroughly checked, cleaned from contamination, degreased, checked for screwing and unscrewing, measured, and weighed. Some of the steel couplings (5 pieces), for a closer approximation to reality, were treated in a phosphate solution, as it takes place during operation. For this, a solution of the following composition was used: phosphoric acid -85 g/l; zinc oxide -17 g/l; sodium nitrate -2 g/l; distilled water -1 liter. The phosphating process was carried out at a temperature of 120 °C for 30 minutes.
The MAO-coating was formed on the samples of 1953T1 aluminum alloy in a weakly alkaline electrolyte with the composition: 2 g/l KOH + 9 g/l Na2SiO39H2O + 4 g/l 6(NaPO3) for 15 and 30 minutes. The thickness of the formed coating was 15-20 and 35-40 microns, respectively.
In addition, since the MAO-coating has a sufficiently high porosity, especially in the surface layer, to reduce this one, some of the aluminum alloy samples (with numbers 12 and 22), after MAO-processing were treated by fluoroplastic suspension (F-4D) and then were dried for 30 minutes in a drying oven at a temperature of 150 °C. Table 1 presents the initial data on the samples before testing. To control the corrosion resistance, two connections of the samples ("pipe" -coupling) were made without MAO-coating. All samples were designated in accordance with the accepted code and then assembled into connections. Samples 0 and 1 were without MAOcoating, while the steel coupling of sample 1 was subjected to phosphating (marking by ph in Table 1 For testing, a 5% sodium chloride solution was used at a temperature of 80 °C, for which, samples dipped into the solution were placed into a drying oven (see Fig. 2).  After exposition in a corrosive environment, the samples were cleaned of corrosion products, measured, weighed, and the results were recorded in the journal.
To observe changes in the surface layer, a Carl Zeiss SIGMA scanning electron microscope with an energydispersive attachment was used.
Changes in the qualitative and quantitative composition of the test materials were evaluated using a portable XRF analyzer DELTA Professional with the new X-Count technology.

Results of research and discussing
The view of the samples after exposition in a corrosive environment is shown in Fig. 3. The results of subsequent measurements and weighing of samples after testing are presented in Table  2. Table 2. Data of samples after tests.
Analysis of surface changes in the contact zone of the samples showed that typical corrosion products in the form of flakes with cracks in the uncoated aluminum alloy samples are predominated (Fig. 4 a, b). At the same time, no such formations are observed on the samples with MAO-coating (see Fig. 4 c, d). Steel couplings practically did not undergo any corrosion damages, except for the appearance of changes of colour and light deposits of corrosion products, which were then cleaned off. At the same time, it should be noticed, to a lesser extent, such changes were typical for the samples subjected to a preliminary phosphating. A more subtle examination of the surface with a scanning microscope (at a magnification of 500x) showed that, indeed, corrosion products on uncoated aluminum alloy samples are quite bright presented (Fig.  5 a and b). The study of the elemental composition of the aluminum alloy samples at the place of contact with the steel coupling on the DELTA Professional device showed that on samples without MAO treatment (samples 0 and 1), the elemental composition differs markedly (see Table 3) from the initial one [12], which according to OST1-92014-90 has the following approximate chemical composition,%: Al -balance, Mg 2.4-3.0; Zn 5.6-6.  Moreover, the content of magnesium in both samples is not detected (while in the original alloy its content is from 2.4 to 3.0%). On the other hand, the copper content increased significantly (more than 2%), in contrast to the initial one -0.4 -0.8%.
In the samples treated by the MAO method (samples 12 and 21), there is a noticeably increased silicon content, which is explained by the presence of siliconcontaining compounds (SiO2 oxides, 3Al2O32SiO2 mullite) in the MAO coating.
The nature of the samples chemical composition change can also be seen in the graphical diagrams of the chemical composition investigation (Fig. 6). compounds in a corrosive environment of sodium chloride during the exposition -corrosion products (see Fig. 5 a, b), which include these elements.
The calculated values of the corrosion rate by weight (g/m 2 h) showed that the presence of a corrosion-resistant MAO-coating in the contact zone reduces the corrosion rate of the aluminum alloy (Fig. 7). The corrosion rate of steel couplings in the joints with a thicker MAO-coating thickness is significantly more. Thus, with a smaller thickness of the MAOcoating, the resistance against contact corrosion is higher.

Conclusion
Analyzing the presented data, it can be concluded that the use of MAO-coating as a protective barrier in the contact zone of a threaded joint of an aluminum pipe and a steel coupling allows reducing the corrosion rate of an aluminum pipe and can be recommended for the use in operation. At the same time, in order to reduce contact corrosion of the steel coupling, the thickness of MAOcoating on the aluminum alloy should be formed as lower as the best.