RU2319139C2 - Method for determining defects of hydro-isolation cover and corrosion damage of external surfaces of underground and underwater pipelines - Google Patents

Abstract

FIELD: flaw detection methods.

SUBSTANCE: method for determining location and area of defects of hydro-isolation cover and depth of corrosion damage of external surfaces of underground or underwater pipelines includes measurement of polarization potential in 5...10 seconds after polarization is removed, along polarization curve by tops and edges of cone dips, area of damage in hydro-isolating cover and depth of corrosion damage are determined using suggested mathematical formulas.

EFFECT: it is possible to determine location and depth of corrosion damage in external surface of underground cathode-protected pipeline, and also area and location of damage in film isolation.

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Description

The invention relates to methods for non-contact determination of defects in a waterproofing coating and corrosion damage to the outer surfaces of underground and underwater cathodically protected pipelines with film waterproofing using electrochemical analysis and can be used in underground pipeline transport.

Known methods for determining the places of corrosion damage to the outer surfaces of underground pipelines, which include the use of in-tube shells — flaw detectors equipped with ultrasonic or magnetic flaw detection devices (V.V. Kharionovsky. Prospects for the development of gas pipeline diagnostics in Russia. // Problems of the resource of gas pipeline structures. M. VNIIGAZ. 1995. C.3 ... 12.).

Significant disadvantages of these methods are

1) the high cost of research in-tube shells;

2) the need for expensive gateway devices on pipelines;

3) the requirement to decommission the gas pipeline during the passage of the in-line flaw detector in the section;

4) the inability to determine the defects of the waterproofing coating;

5) the use of in-pipe shells is possible only in 50% of gas pipelines, since half of the gas pipelines have unequal fittings.

Known methods for determining the defects of the walls of underground pipelines using diagnostic tools for external control, allowing early diagnostics, consisting in the application of acoustic emission and ultrasonic testing (V.V. Kharionovsky. Prospects for the development of gas pipeline diagnostics in Russia. // Resource problems for gas pipeline structures. M VNIIGAZ. 1995. C.3 ... 12.).

Among the disadvantages of the methods include the following:

1) the need for contact with the surface of the underground pipeline, which requires the implementation of significant amounts of expensive earthwork and almost completely eliminates the possibility of applying these methods in flooded and swampy areas where the pipeline is laid below the groundwater level and at underwater crossings;

2) the proposed methods do not allow to determine the defects of the waterproofing coating of an underground structure.

Closest to the proposed invention is a method for determining defects in the insulation coating of underground and underwater pipelines by cathodic polarization of the pipeline, measuring its potential and finding the location and size of defects in the insulation coating by changing the measured potential value (AS No. 873097 G01N 27/26), which, in order to improve the accuracy of detection of defects by eliminating the influence of heterogeneity of soil conditions on the measurement results, before measuring the potential remove the cathode floor polarization and the measured speed measurement value of the pipeline capacity is judged on the magnitude of the defects.

The disadvantages of the proposed method are

1) low accuracy of determining time intervals in the field due to the high speed (from 10 ^-4 to 1 ... 2 s) of establishing electrochemical equilibrium between the electrode in the soil volume and the electrolyte located in the defect zone in the case of a small diffusion layer thickness and high speed diffusion;

2) the inability to determine the location and depth of corrosion damage to metal pipelines.

The objective of the invention is the determination of the places and depth of corrosion damage to the outer surface of the underground cathode-protected pipeline, as well as the area and places of damage to the film waterproofing.

The problem is achieved by the method of determining the area and location of defects in the waterproofing coating and the depth of corrosion damage of the outer surfaces of underground and underwater pipelines by cathodic polarization of the pipeline, measuring its potential and finding the location and size of defects by changing the potential, and the polarization potential is measured in 5 ... 10 s after removal of polarization; of the polarization curve of the vertices and edges of the "funnel dips" is defined area S [mm ^2] waterproofing coating damage by the formula

S = 40000Е _{EMF (CORR)} / ΔL,

where E _{EMF (CORR)} = E _PA -E _PC [V],

E _PA ^{- the} potential of the anode zone;

E _PC ^{- the} potential of the cathode zone;

ΔL is half the length of the cathode zone.

Also, the depth δ _(CORR) [mm] of corrosion damage is determined from the tops and edges of the "funnel of dips" according to the formula

δ _(CORR) = 2 [E _{EMF (CORR)} / ΔL] (T _G -5) [I _MAX / I _{D (SR)} ],

where T _G - pipeline service life in years;

I _MAX - the maximum possible (passport) current of the cathodic protection station [A];

I _{D (SR)} is the average effective current of the cathode station during the operation period [A].

New significant features:

1) the measurement of the polarization potential is carried out 5 ... 10 s after removal of the polarization;

2) the damage area of the waterproofing coating of the underground structure is determined by the tops and edges of the "crater of failure" of the polarization potential (cathode zone) on the polarization curve,

3) the width of the craters of the dip is judged on the extent of the defects of the waterproofing coating;

4) the zones of corrosion destruction of the metal are determined along the edges of the "funnel of failure" (anode zones of destruction) on the polarization curve;

5) the magnitude of the damage is judged by the gradient of potentials along the length of the pipeline along the tops and edges of the "crater of failure".

The above new essential features, together with the known ones, provide a technical result in all cases to which the requested amount of legal protection applies.

Obtaining the technical result of the invention is achieved by the fact that the extreme potential difference of the two sections of the underground steel structure is determined by the polarization curve, which indicates that there are favorable conditions for intense corrosion processes on the anode surfaces having a lower electrode potential. Theoretically, cathodic protection effectively slows down the corrosion of metal, as shown by the measurement data of the total potential of the pipe surface - the entire pipeline has a total potential much lower than the polarization potential of the metal of the pipe. As a rule, the potential difference on the curve of the total potential of the surface of the steel structure along the length of the underground pipeline is at least an order of magnitude lower and rarely exceeds 0.05 V, but, nevertheless, corrosion damage is observed on the anode surfaces of 2 cathodically protected underground structures, places the location of which is easily identified by the curves of polarization potentials (figure 1).

The causes of intense corrosion processes of pipe steel even in the immediate vicinity of the drainage points of the cathodic protection stations are determined by the operating mode of the thyristor regulators of the cathodic protection stations, the current and voltage in which are determined by the ratio of the current flow in the circuit and the pause time between individual pulses, since the thyristor regulators of cathode stations is based on the phase-pulse principle, according to which the required value of the effective protective current and voltage Poles are obtained by converting an alternating sinusoidal single-phase current of industrial frequency 50 Hz to unipolar current pulses I _AND voltage U _{AND of} variable amplitude and duration (depending on the magnitude of the required effective protective voltage U _D and current I _D ), with a frequency of 100 Hz (Fig. .2). This leads to the fact that in the anode sections 2 (figure 1) with a lower electrode potential E _PA formally protected by the effective value of the protective potential U _D underground pipeline for a significant part of the time (in the pauses between individual protective current pulses I _ZI ) is free from protective the action of the cathodic protection current, as a result of which between the cathode 3 and the anode sections 2 appears E _{EMF (CORR)} = (E _PA- E _PC ) EMF corrosion and from the anode section 2 (figure 1) the corrosion current (figure 2) I _CORR , which is protective for the cathode portion I _{K RR} = I _ZEDS, carries metal ions from the steel pipe. The operation diagram of the thyristor station cathodic protection of the underground gas pipeline is shown in figure 2.

Intensive corrosion processes 5 (Fig. 3) of the metal of the pipe 6 occur under the waterproofing carpet 7 in those parts of the pipe surface where the waterproofing has no external signs of mechanical damage with violation of the integrity of the waterproofing carpet, except for its slight local peeling. The length of the channels 8 (formed under the film coating as a result of its peeling) usually does not exceed several meters, and the cross-sectional area of the space formed under the waterproofing carpet does not exceed fractions of a square centimeter and is filled with electrolyte - groundwater, and the channels themselves communicate with areas where there are significant mechanical damage to the integrity of the protective carpet 9. In places of damage to the insulation, the surface of the pipe is aerated with air in the ground and dissolved in electro ite - groundwater, as a result of the pipeline on the surface portions of the metal appear, adsorption of hydrogen which is difficult because of the presence of significant amounts of oxygen and nitrogen in the air. At the same time, in the immediate vicinity of the zones of powerful local damage to the waterproofing, there are zones of delamination of the film coating, filled with electrolyte, the access of oxygen to which is difficult. Under the action of cathodic polarization, the near-cathode layer of the electrolyte is actively alkalized, but on surfaces with strong local damage to the film waterproofing due to the openness of the system, the pH value of the near-cathode layer decreases quite quickly to pH = 7. In places of local minor delamination of the waterproofing, where communication with the environment is difficult, the pH value remains high for quite a long time: pH = (9 ... 12). As a result, a concentration galvanic cell begins to operate, the electromotive force E _{EMF (CORR)} [B] of which is determined by the potential difference between the anode E _PA and the cathode E _PC zones, depending, in accordance with the Nernst equation, on the pH values of the corresponding sections of the underground pipeline and the logarithm of the relations amounts of adsorbed hydrogen in the cathodic zone is well aerated surface _K powerful field insulation failure to the amount of hydrogen adsorbed with _a deposits with fine ano hydrochloric zone in places of local slight local film peeling waterproofing covering:

E _{EMF (CORR)} = -0.0592 [ΔpH + log (s _K / s _D )].

The consumables of this galvanic cell are hydrogen and iron pipes, since the potential difference activates all the oxidation processes. At the moment of receipt of negative protective impulses to the tube from the cathodic protection station, the potentials of these sections are partially equalized due to polarization. As a result, corrosion processes at the anode sites are greatly slowed down, but during periods of pauses between negative protective impulses, corrosion processes resume.

When assessing the state of an existing gas pipeline with a service life of more than 5 years, an empirical expression is proposed that allows us to estimate the depth of corrosion damage δ _(CORR) [mm] of the pipe wall

δ _(CORR) = 2 (E _{EMF (CORR)} / ΔL) (T _G -5) (I _MAX / I _{D (SR)} ),

where E _{EMF (CORR)} / ΔL [V / m] is the potential gradient between the top and base of the "funnel of failure" 4 in section 3 of the polarization potentials curve (Fig. 1);

T _G - gas pipeline service life in years;

I _MAX - the maximum possible (passport) current of the cathodic protection station [A];

I _{D (SR)} is the average effective current of the cathode station during the operation period [A].

Corrosion damage to the pipe surface is the greater, the more the surface of the underground steel structure is exposed, since the value of the corrosion current (which determines the depth of corrosion damage) determines the area of the exposed (cathode) zone. The length of the exposed sections of the pipeline is equal to the length of the cathode zones and is determined by the curve of polarization potentials. In section 1, the corrosion damage is insignificant, since the potential gradient in the section is insignificant (Fig. 1). Area S [mm ² ] damage to the waterproofing coating is determined by the formula

S = 40000Е _{EMF (CORR)} / ΔL

Example. On the polarization curve of the underground pipeline there is a section with a length of ΔL = 10 m, on which the surface potential of the pipe has changed by the value of E _{EMF (CORR)} = 0.2 V. The maximum possible (passport) value of the current of the thyristor cathodic protection station is I _MAX = 100 A, the average current value according to the operational log of the cathode station for 11 years of its operation was I _{D (SR)} = 10A. Depth of corrosion damage δ _{(CORR) of the} surface of the anode zone of the pipe:

δ _(corr) = 2 (Е _{EMF (CORR)} / ΔL) (Т _Г -5) (I _MAX / ID _(СР) ) = 2 (0.2 / 10) (11-5) (100/10) = 2.4 mm;

area S [mm ² ] of damage to the waterproofing coating will be

S = 40,000 E _{EMF (CORR)} / ΔL = 40,000 · 0.2 / 10 = 800 mm ² .

The list of items in the drawings

Figure 1.

1 - areas of minor corrosion damage;

2 - anode (destroyed by corrosion) surface sections of cathode-protected underground structures;

3 - cathode surface areas of cathodically protected underground structures;

4 - “funnel of failure” of polarization potential;

E _PC - polarization potential of the cathode sections of the pipe;

E _PA - polarization potential of the anode sections of the pipe;

Figure 2.

U _AND , I _AND - voltage and current of the protective pulse of the cathode station;

U _D , I _D - the effective values of the protective voltage and current of the station;

I _ZI - pulse protective current of an underground structure;

E _PC - polarization potential of the cathode sections of the pipe;

E _PA - polarization potential of the anode sections of the pipe;

E _{EMF (CORR)} = (E _PA -E _PC ) - EMF corrosion of the anode sections of the pipe;

I _KORR = I _ZEDS - corrosion current of the anode sections created by the E _{EDR (KORR)} equal to the protective current of the cathode sections created by the same EMF.

Figure 3.

5 - corrosion deposits on the surface of a steel pipe;

6 - steel pipe;

7 - peeled off from the outer surface of the steel pipe waterproofing coating;

8 - channel filled with groundwater - electrolyte, formed under a film coating as a result of delamination of the waterproofing;

9 - violation of the continuity (damage) of the waterproofing.

Claims (1)

A method for determining the location and area of defects of a waterproofing coating and the depth of corrosion damage of the outer surfaces of underground and underwater pipelines by cathodic polarization of pipelines, measuring its potential and finding the location and size of defects by potential change, characterized in that the polarization potential is measured in 5 ... 10 after removal from the polarization of the polarization curve in the vertices and edges define funnel dips area S [mm ^2] gidroizo damage yatsionnogo coating according to the formula S = 40000Е _{EMF (CORR)} / ΔL, where E _{EMF (CORR)} = E _PA -E _PC [V], E _PA - the potential of the anode zone, E _PC - the potential of the cathode zone, ΔL - half the length of the cathode zone [m], the depth δ _(CORR) [mm] of corrosion damage is also determined from the tops and edges of the funnels of dips by the formula δ _(CORR) = 2 ( _{EED (CORR)} / ΔL (T _G -5) (I _MAX / I _{D (SR)} where E _{EMF (CORR)} = E _PA -E _PC [V], ΔL - half the length of the cathode zone [m], T _G - gas pipeline service life in years; I _MAX - the maximum possible (passport) current of the cathodic protection station [A]; I _{D (SR)} is the average effective current of the cathode station during the operation period [A].

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