In this paper the advantages of corrosion monitoring are described using gravimetric witnesses samples for oil production and refining companies, the necessity and effectiveness of their use are founded, the additional capabilities of the method are shown, especially control of deposits on the internal surfaces of equipment and their effect on the corrosion rate. The paper presents data on the coupons geometric form, it is noted that witnesses samples of rectangular shape are the most widespread. This article also presents information about the modernization of coupon holders for existing CDU VDU – 4 of JSC “Gazprom neftekhim Salavat”, the advantages of suggested design are described, data of corrosion monitoring on the unit since December, 2013 are provided. Noted, that to hold a valid comparison of different coupons characteristics in an industrial environment is not possible, due to the lack of coupon holders substitutability on different corrosion control sites of unit. In this regard, comparative tests of coupons were conducted in the laboratory, coupons selected for the tests were rectangular, disk-shaped and rectangular with a weld, and the basis for coupon selection is shown. Lab installation was prepared for tests which scheme is given. All samples were made from the same material – BSt3sp by GOST 380 – 2005. Tests were carried out in 50% of mass water solution of lemon acid, duration of each series of tests were 3, 6 and 9 hours. By the tests results the mass loss of samples was defined and mass and deep indicator of corrosion rate was calculated. On the basis of obtained data, diagrams of corrosion rates dependence on the sample witness type and tests time were made. The received results showed that coupons with an additional source of residual internal stresses in the form of weld are exposed to more serious corrosive attack (show greater tendency to corrosion) in comparison with rectangular and disk-shaped coupons. The explanation of revealed regularity is given in the article using data on macro and micro electrochemical heterogeneity of welded connections and the doctrine about electrochemical potentials. Thus, using samples witnesses that have additional source of internal stresses such a weld, for corrosion monitoring make it as close as possible to the actual conditions.