During technical diagnostics of steel tanks in operation, ultrasonic testing of the base metal and welded joints is mandatory. The surface of the tanks has an anti-corrosion paint coating, which, according to the traditional approach, must be removed for ultrasonic testing.

The need to remove the protective coating is due to its influence on defect detectability. Ultrasound interference in a coating layer of variable thickness leads to sensitivity oscillations and, consequently, incorrect assessment of the echo signal amplitude from discontinuities in the metal, leading to under-rejection of critical defects or over-rejection of acceptable defects.

Removing and restoring protective coating after inspection requires significant material costs and organizational efforts involving specialized organizations. When using traditional ultrasonic testing technology for welds and tank wall metal, the costs for surface preparation (cleaning) for inspection and for restoring the protective coating of a 20,000 m³ oil storage tank can reach 4-6 % of the initial tank cost.

The use of modern paint and varnish materials guarantees reservoir protection against corrosion for 15-20 years, while regulatory documents (for example, RD-16.01-60.30.00-KTN-063-1-05 OJSC «AK «Transneft») stipulate diagnostics with a periodicity of 4-5 years. Under these conditions, questions arise: how to conduct technical diagnostics of reservoirs that are still under warranty for their coating? Is it possible to ensure comprehensive ultrasonic testing without removing the anti-corrosion coating that is still under warranty?

Experimental studies have been conducted to answer these questions regarding the effect of paint coatings on ultrasonic testing parameters (sensitivity, incident angle) depending on the type and thickness of the coating.

The nomenclature of the applied coatings is very broad. Therefore, during the research, all coatings were divided into seven groups depending on the base of the paints and varnishes used:

Experiments have revealed how the thickness of an anti-corrosion coating affects the detectability of defects in the wall of a storage tank. The research was conducted on a representative sample of coatings, considering their types and bases.
Figure 2 shows an example of experimental research results on the influence of paint coating thickness and type on inspection sensitivity, obtained for a frequency of 5.0 MHz and an incident angle of 60 degrees (for the shear wave case). The obtained relationships allow for adjusting the sensitivity of the flaw detector when inspecting welds and tank metal. Similar relationships were obtained for incident angles of 45 and 70 degrees and corresponding ultrasonic oscillation frequencies of 2.5 and 5.0 MHz.

These and similar dependencies have enabled the development of an approach to ultrasonic control of the wall without removing the anti-corrosion coating. The basis of this approach is the selection of control parameters for which the influence of the coating thickness is minimal, and the introduction of a series of correction factors that take into account the coating properties.

Control of welded joints through anti-corrosion coating is performed in 4 stages, in the sequence shown in Figure 3.

Stage 1: Visual and dimensional inspection of the welded joint and anticorrosive coating in the area of piezoelectric transducer (PET) movement is being carried out.
Stage 2: UT is performed at a sensitivity increased by 6-10 dB.
Stage 3: The thickness of the anticorrosive coating is measured in the area where the UT probe is located when a defect is detected.
Stage 4: The results of ultrasonic testing (signal amplitude and conditional length) are corrected depending on the thickness and type of coating.

The technological procedure for ultrasonic inspection of tank wall welds described above is implemented using any general-purpose ultrasonic flaw detector equipped with standard piezoelectric transducers with incident angles of 45, 60, and 70 degrees and a working frequency of 2.5 and 5.0 MHz.
This technology formed the basis of the developed standard RD-19.100.00-KTN-545-06 «Ultrasonic inspection of the wall and welded joints during operation and repair of steel vertical tanks» and was implemented in the system of OJSC AK Transneft.