Tubing is one of the main types of pipes used in the oil and gas production industry. One of the characteristics of modern oil and gas production is the tendency to tightening conditions of operation of down-hole equipment including pipe strings which are exposed not only to mechanical stresses (stretching, twisting, bending, etc.) but also to the intense influence of aggressive media.
It is known that the main cause of rapid failure of pipes is corrosion which drastically reduces their service life and cuts production volumes which ultimately leads to a decrease in profitability of oil and gas production. The most common causes of accidents with black (uncoated) tubing are corrosion, thread wear and fatigue failure. This fact requires a significant extension of the scope of works connected with execution of down-hole operations.
The accidents resulted from corrosion-erosion failures of oilfield pipes lead not only to losses of a large number of pipes, equipment downtime and incomplete oil extraction but also to irreversible pollution of the environment with oil and sewage.
Therefore, development of methods for effective protection of pipes against aggressive media is timely and relevant.
Fig. 1 shows typical appearance of corrosion failure caused by perforation pitting corrosion of the threaded end of the tubing with transition to the pipe body
One of effective ways to increase corrosion resistance and operational reliability of pipe products is their protection with the help of various anti-corrosion coatings. In the case of pipes with threaded ends, the selected coating should not only protect against corrosion but also ensure a high wear resistance and tightness of “pipe-coupling” threaded connections.
The volume of production of such pipes is constantly growing in the world including pipes produced by enterprises of the oil and gas industry.
Analysis of the currently used corrosion protection coatings, numerous studies and the experience gained in operation of tubing in oil and gas fields shows that diffusion zinc coating is one of the most effective types of coatings for corrosion protection of tubing. In comparison with galvanic and phosphate coatings, they have higher hardness and wear resistance. Such coatings are less susceptible to various mechanical damages and have a high degree of adhesion to the steel surface of the pipe due to diffusion bonding.
Diffusion coatings, unlike galvanic, phosphate and polymer coatings which just shield surface of the protected product from the effects of corrosive media, form not only a protective layer on the product, but also penetrate into its surface layers strengthening them by formation of iron-zinc intermetallides. At the same time, physical and chemical properties of metal products, corrosion resistance and other performance characteristics improve greatly. This, first of all, minimizes degradation of protective properties of the coating subjected to mechanical influences during pipe transportation, storage and operation of metal products. The unique properties of the diffusion zinc coating are due to its structure which ensures good plasticity and protective properties while performing a role of solid lubricant which is especially important for threaded joints of pipes, their operational reliability and durability.
An important indicator of quality of the diffusion zinc coating is uniformity of coating distribution on the profile of the threaded tubing end and the coupling which provides the threaded joint with a high tightness and operational reliability. The mentioned complex of properties of the diffusion zinc coating ensures improvement of the pipe threading resistance and eliminates loss of tightness of threaded joints at multiple (up to 20-30 or more times) “screwing-unscrewing” operations.
Such coating characteristic as microhardness is especially important since it largely determines performance characteristics of diffusion-galvanized tubing when they work in corrosive and erosive media. Tests of threaded joints of diffusion galvanized tubing and their couplings confirmed their high operability (wear resistance) and preservation of their properties after repeated screwing-unscrewing of pipes and couplings.
Diffusion galvanized pipes can easily be transported without risk of damaging the coating which is difficult to obtain for pipes with non-metallic coatings, especially enamel or glass-enamel coatings.
The use of intensive methods for developing oil fields, increased watering of well products because of pumping large volumes of highly mineralized brine and waste water into productive strata to maintain pressure in them sharply increased corrosion of pipes which has caused the need of improved performance characteristics of protective coatings.
Study of the mechanism of formation of diffusion zinc coatings and a wide range of scientific and practical works made it possible to develop a diffusion zinc coating of a new generation, Delta 5+, and improve technology of its application to oil-country pipes.
For example, in the classical diffusion zinc coating, two phases are mainly observed: a G phase (“gamma” phase) containing up to 28% of iron which is an intermetallic compound in a thin (2-4 μm) layer directly on the surface of the coated product and a δ-phase (“delta” phase) under this layer which is an intermetallic compound as well with iron content of 7 to 11.5%.
An important characteristic of the new generation coating (Delta 5+) is high corrosion resistance and uniformity of its distribution on the pipe surface and, what is especially important, on the thread profile. Metallographic studies have established that the layer of the new diffusion zinc coating Delta 5+ is evenly distributed on the thread of coupling and nipple end of tubing which ensures high tightness and operational reliability of threaded joints.
The above mentioned complex of properties of the diffusion zinc coating ensures higher (up to 20-30 or more times) durability of threaded pipes and excludes loss of tightness of threaded joints in multiple “screwing-unscrewing” operations taking place in construction, operation and repair of wells.
At present, one of the up-to-date processes of diffusion galvanizing is the process developed by Neozinc LLC (Moscow, Russian Federation). Furnaces, containers and other equipment of Neozinc technology ensure zincing both tubing and long products (up to 12.5 m in length) and small parts of various shapes and lengths that are supplied with long products, for example, couplings for tubing and casings. According to available information, containers and cassettes for various types of products are designed and manufactured in such a way as to minimize use of manual labor during their loading into and unloading from the furnace for chemical and thermal treatment. An electric furnace of a special design has a reliable multi-zone electronic control system for maintaining temperature conditions of the galvanizing process and ensuring a high uniformity of product heating. It makes it possible to obtain a diffusion zinc coating of high-quality and uniform thickness along the entire length of the zinced products.
One of the most effective ways to increase corrosion resistance and extend service life of tubing is their protection with coatings of a “duplex system” type.
Such coatings are a combination of a metal coating (zinc or iron zinc alloy) and one or more layers of paint or powder coating. The presence of metallic and non-metallic coatings in the duplex system ensures optimal anti-corrosion properties of the steel product in aggressive environments due to a combination of the electrochemical protective effect of the zinc coating with the waterproofing effect of the paint coating. The degree of protection of the duplex system is higher than the sum of individual degrees of protection of the zinc coating and the polymer coating (paint).
In the “duplex system” coating, barrier and protective techniques of metal protection were combined for the first time by uniting several most effective materials and the methods of their application complementing each other in the resulting multifactor system.
A “duplex system” with the trade name Majorpack developed by specialists of DNA Engineering Co., Italy, in cooperation with Neozinc Technology JSC, Russia, has found an extensive use for corrosion protection of tubing.
The Majorpack protection system consists of an intermetallic iron-zinc layer applied to the inner and outer surfaces of the tubing as well as threaded joints and tubing couplings by diffusion galvanizing. A multicomponent polymer (barrier protection) is applied on this layer. There are also purely polymeric coatings implementing just the barrier method of protection. Composition of the multicomponent polymer is selected for specific well conditions to provide protection against chemical and acid corrosion as well as protection against flow phenomena. It has good hydrophobic properties, reduces likelihood of formation of paraffin-asphalt resinous deposits (PARD). Structural diagram of tubing with a “duplex-system” coating is shown in Fig. 2. Tubing with such coating has high corrosion resistance and a long service life.
The intermetallic iron-zinc (protective) layer protects the tubing against electrochemical, subfilm and pitting corrosion providing electrochemical protection of the tubing surface as well as protecting the pipe body in the event of damage of the barrier layer. It also serves as a primer for the barrier layer. The protective layer is characterized by high adhesion (more than 30 MPa) and microhardness (2.5 times higher than that of steel grade 80), increased resistance to physical damage.
Development of new protective coating systems for oil-field pipes operated in severe corrosive conditions of oil and gas extraction was the subject of scientific and practical works of researchers and designers from:
- State Enterprise “Ya.Ye. Osada Scientific Research Tube Institute”, Dnipro, Ukraine (research and field tests);
- Ukrainian State University of Chemical Technologies, Dnipro, Ukraine (polymeric materials),
- Prydniprovska State Academy of Civil Engineering and Architecture, Dnipro, Ukraine (metallurgy and heat treatment),
- Neozinc Technology JSC, Moscow, Russian Federation (research and industrial testing of pipes).
The result of this cooperation was development of an improved diffusion metallization process that makes it possible to obtain protective diffusion zinc coatings with high performance characteristics and ensures meeting of special requirements consisting in a strict control of thickness and uniformity of the coating layer on the surface of the protected product, especially in location of the pipe and coupling threads. This made it possible to expand field of application of diffusion zinc coatings and their use in protection of steel products operated in complicated corrosion-erosion conditions.
The generalized results of the researches carried out during development and industrial testing of tubing with various protective coating systems are presented in monograph Diffusion Zinc Coatings: Properties and Fields of Application. Reference Book. By Ye.V. Proskurkin, V.A. Gelovani, A.N. Sonk, /edited by Ye.V. Proskurkin and D.А. Sukhomlyn /Moscow, Nauka Publishers, 2017 (Fig. 3).
Particular attention in the book is given to various systems of protective diffusion zinc and combined coatings in the oil and gas industry. Data are given not only for corrosion of oil country pipes and tubes and their threaded joints in aggressive hydrogen sulphide and carbon dioxide media but also the protective capacities of diffusion zinc coatings applied to threaded joints.
Thus, protective coating systems that are a combination of a metal coating (zinc or iron/zinc alloy) and one or more layers of paint or powder coating are promising for tough operating conditions of pipes in oil and gas production.
In such systems, barrier and protective methods of metal protection are united in a combination of several most effective materials and methods of their application complementing each other in the resulting multifactor system.