RU2476621C2 - Corrosion protection method of welded steel structure - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: corrosion resistant coating (CRC) made in the form of a solid electrically conductive layer is applied. Electrically conductive layer is applied along the weld and made from high-electroconductive material with the width, which is determined with the ratio of H_CRC=B+2L_HAZ+δ, where B - weld width, L_HAZ - width of heat-affected zone (HAZ) on one side of the weld, δ - thickness of parts of welded steel structure. Prior to application of the electrically conductive layer, surface of parts on the reverse side of the weld is cleaned. In particular cases of the invention implementation, the solid electrically conductive layer is applied by sputtering or obtained from copper.

EFFECT: providing effective long-term and high-technology corrosion-resistant protection of welds and heat-affected zones of parts of welded steel structures.

3 cl, 2 dwg

Description

The invention relates to methods for protecting against corrosion of marine objects of general purpose engineering - ships, stationary and floating structures and structures.

Various corrosion protection are known [2, p. 277-281]. Among them, the most widely used are electrochemical (cathodic and tread) methods of electrochemical protection (ECP) against corrosion in combination with paint and varnish anticorrosive coatings.

The closest in technical essence to the proposed method of protection is comprehensive protection against corrosion and fouling (options), including applying a continuous conductive layer of anticorrosive and antifouling coatings, measuring the speed of sea water and the specific transient resistance of the anticorrosive coating and based on them the implementation of cathodic polarization of the housing [one].

The method is difficult to implement and has the following disadvantages:

- it does not protect the numerous welds and the heat-affected zone of marine objects of general purpose equipment from electrochemical corrosion associated with the Seebeck effect (due to the presence of thermoelectromotive force between the welded parts in conditions of good electrical conductivity of sea water). Since two parts, even made of the same steel, are necessarily different in chemical compositions, and the zonal temperature of sea water almost always differs slightly from the temperature of the ship’s hull, welding of these parts leads to the formation of a thermocouple that accelerates the corrosion process many times under conditions of conductive sea water seam and heat affected area. The same effect is observed due to differences in welded parts in the metal structure, the presence of residual deformations and stresses [6, p. 211];

- requires the application of anticorrosive (PCP) and electrically conductive (EPP) coatings to the entire underwater part of marine equipment, which is quite a costly, highly labor-intensive operation;

- requires the installation of auxiliary electrodes (E1-E4 ...) and reference electrodes (ES);

- the electrodes come into direct contact with seawater and, under conditions of electric current flowing through them, are themselves subject to electrochemical corrosion;

- requires the installation of special sources of polarization (IP) - as a rule, automatic direct current sources, ensuring the maintenance of a given protective potential or protective current density;

- requires the use of switchboards (RC) and the connection through them of the negative pole of the FE to the structure K or (and) to the electrodes of the E, and the positive pole - using the contact electrode of the EP to the EPP and (or) to the electrodes E.

At the same time, since welding is the main way to permanently connect the hull elements of marine objects of general purpose equipment, the numerous welds and their heat-affected zones that are in direct contact with high-conductivity sea water are constantly exposed to the aggressive effects of electrochemical corrosion. The wear rate of the welds is extremely high - reaches 1.0-3.0 mm / year [2, p. 211], while the corrosion rate of the outer skin in the underwater part of the ship’s hull is much lower and does not exceed 0.19 mm / year [2, Table 13 on p. 211].

The technical result of the invention is an effective, long-term and high-tech anti-corrosion protection of welds and heat-affected zones of parts of welded metal structures.

In the proposed method of corrosion protection of welded metal structures, including applying an anti-corrosion coating, made in the form of a continuous conductive layer, additionally, the conductive layer is made of highly conductive material and is applied in a width that is determined by the ratio

N _PKP = V + 2L _HAZ + δ,

where B is the width of the weld, L _HAZ is the width of the heat-affected zone on one side of the weld, δ is the thickness of the part of the welded metal structure along the weld, and before applying the electrically conductive layer, the surface of the parts is cleaned from the back of the weld, the coating is sprayed and obtained from copper.

Figure 1 shows the arrangement on the welded metal structures of anticorrosion coating.

Figure 2 shows the distribution of electric currents in metal, sea water and a layer of anticorrosion coating.

In figure 1 is indicated: 1 - details of welded metal structures; 2 - weld; 3 - the outer (front) side of the weld, in direct contact with sea water; 4 - the reverse side of the weld, not in contact with sea water; 5 - heat-affected zone on the outside of the weld; 6 - heat-affected zone from the rear side of the weld; 7 - sea water; 8 - a layer of anticorrosion coating; δ - details of welded metal construction; B is the width of the seam on the front side of the weld; B + 2L _HAZ - width of the zone of formation of thermopower, consisting of the width of the weld and the width of two heat-affected zones; N _PKP - the width of the layer of anti-corrosion coating.

In figure 2 is indicated: 1 - details of welded metal structures; 2 - weld; 5 - heat-affected zone on the outside of the weld; 7 - sea water; 8 - a layer of anticorrosion coating; 9 - equilibrium current lines in sea water; 10 - equilibrium current lines in metal structures passing through a layer of anticorrosive coating; 11 is a current vector in the heat-affected zone passing through a layer of anticorrosive coating.

As follows from figure 1, a layer of anticorrosion coating is applied not on the outer (front) side of the weld, which is in direct contact with seawater, but on the opposite, reverse side of the weld. The meaning of the coating is not to exclude the contact of the weld with an aggressive marine environment, but to minimize the main cause of increased electrochemical corrosion of the metal structure material - electric currents flowing through the surface of the weld and heat-affected zone in the direction of sea water under the effect of the Seebeck effect. This decrease in electric current is achieved by shunting the current lines flowing through seawater with another highly conductive electric circuit through a layer of anticorrosion coating. Thermoelectric power arising under the Seebeck effect in this local volume of the weld and the heat-affected zone is a current source for two parallel electrical circuits: one circuit through seawater (stream lines 9, figure 2), the other through a layer of anticorrosive coating (lines current 10-12, figure 2). In this case, the currents through sea water and through a layer of anticorrosion coating will be distributed in proportion to the conductivities of these two circuits.

Given the rather low electrical conductivity of seawater (electrical conductivity is 4.28 S / m at 15 ° C and 5.30 S / m at 25 ° C [3]) in comparison with the electrical conductivity of the anti-corrosion coating layer (58100000 cm / m at 20 ° С [4]), for example, from copper, the strength of the current through seawater will be reduced several times. This, in turn, will drastically reduce the corrosion rate of the weld and heat-affected zone.

The application of a continuous electrically conductive layer of anticorrosive coating to the seam and the heat-affected zone provided reliable protection against corrosion along the entire length of the seam.

The introduction of newly introduced features in the claims has provided the following advantages of this invention compared to the known.

The use of a coating with a width equal to the width of the outer side of the weld and its heat-affected zone, increased on both sides of the weld by the thickness of the part of the welded metal structure, made it possible to reliably shunt electric currents passing through sea water due to the Seebeck effect.

The application of a layer of anticorrosion coating in the direction along the axis of symmetry of the weld on the surface of the parts from the back of the weld allowed to bypass the whole set of individual (elementary) thermoEMFs arising in each cross section of the weld and the heat-affected zone.

The application of a layer of anticorrosion coating on a previously cleaned and degreased surface of the parts ensured a reliable connection (by fusion, gluing, deposition, etc.) of the anticorrosion coating layer with the surface of the weld, heat-affected zone and metal construction parts.

The application of a layer of anticorrosion coating on the back of the weld allowed, firstly, to avoid corrosion of the anticorrosion coating itself, and secondly, to draw the current lines from the outer surface of the weld into the seam, and thirdly, to provide better conditions for the subsequent repair of hulls.

The value of the width of the layer of anticorrosive coating N _PKP defined by the expression

N _PKP = V + 2L _HAZ + δ,

where B is the width of the weld, L _HAZ is the width of the weld zone on one side of the weld, δ is the thickness of the part of the welded metal structure, it provides the effectiveness of corrosion protection of the weld and weld zone, since, on the one hand, it captures the main part of the shunt currents passing through sea water due to the Seebeck effect. On the other hand, unnecessary costs of expensive material of the anti-corrosion coating layer are eliminated.

The implementation of an anti-corrosion coating in the form of a layer of highly conductive material made it possible, firstly, to reduce the cost of corrosion protection of marine objects of general purpose, secondly, to implement it with simple high-tech means, and thirdly, to make the protection effective.

Applying a layer of anticorrosion coating by spraying allowed to expand the technological capabilities of the method - to produce corrosion protection in the vertical direction at the final stage of the manufacture of the ship's hull, or during the repair of the ship.

Obtaining an anticorrosive coating from copper made it possible, firstly, to reduce the thickness of the anticorrosion coating layer without compromising the quality of protection, and secondly, to increase its efficiency due to the higher conductivity of copper in comparison with the vast majority of structural materials.

The method is as follows.

On a pre-welded metal structure by known methods, a narrow strip is cleaned under a layer of anticorrosive coating on the reverse side of the seam with a width equal to the width of the outer side of the weld and its heat-affected zone, increased on both sides of the weld by the thickness of the part of the welded metal structure. When this is used, for example, portable cleaning plants with abrasive wheels. A similar method of rough cleaning the surface provides it with the high roughness necessary for strong adhesion of the applied layer of anticorrosion coating to the base metal (i.e., the metal of the metal structure). In some cases, to provide an additional level of roughness, sandblasting or shot blasting is indicated. Then the cleaned surface is degreased by known methods. For example, the cleaned surface with the help of spray guns is pre-washed with hot water to remove any remaining sand or dust. Then spray the surface is treated with a solution of caustic soda. After which the surface is finally washed with hot, and then with cold water.

One of the known methods is applied to a treated narrow strip a layer of an anticorrosive coating of highly conductive material. So, copper is used as a highly conductive material. The coating is applied by spraying, or other known method. For example, by flame spraying with a CASTO DYN SYSTEM 2000 from Castolin + Eutectic. It is possible to spray with plasma devices, for example, UMP-1 and UMP-2. Other methods of coating include surfacing. For example, mechanized powder plasma surfacing with highly conductive material.

Good results are obtained by soldering on the back side of the seam of the foil of highly conductive material with a width equal to the width of the outer side of the weld and its heat-affected zone, increased on both sides of the weld by the thickness of the part of the welded metal structure. For example, using gallium as solder. Gallium, which is in liquid state at room temperature, is squeezed out of the tube onto the surface of the foil of the anticorrosion coating from a highly conductive material. The foil is rolled by a roller in the back of the seam. After rolling, the foil is almost instantly soldered to the base metal of the metal structure. Moreover, due to the active development of diffusion processes in the soldering zone, the desoldering temperature approaches the solidus temperature of the highly conductive material.

In certain cases, on the reverse side of the seam, a foil of highly conductive material is glued with a width equal to the width of the outer side of the weld and its heat-affected zone, increased on both sides of the weld by the thickness of the part of the welded metal structure. Moreover, for this purpose, highly conductive adhesives are used, for example, "Octopus" type. In the latter case, the bonding of the foil is accompanied by heating of the bonding area. For heating use a blowtorch or a gas flame burner.

In rare cases, metallization and other methods (cold welding, spot and seam contact welding, electric riveting, etc.) are used to apply a layer of anticorrosive coating from a highly conductive material.

The width of the layer of anticorrosive coating N _{PCP is} found from the following expression:

where B is the width of the seam on the front side of the weld;

L _HAZ - the width of the heat-affected zone (i.e., on one side of the weld);

2L _HAZ - width of two heat-affected zones;

B + 2L _3TB ^_ width of the zone of formation of thermopower;

δ is the thickness of the welded metal part.

The increase in N _PCP above the values determined by formula (1), leads to unreasonable losses of expensive anti-corrosion coating. The decrease in N _PCP below the values defined by formula (1), leads to a decrease in the effectiveness of corrosion protection.

The method is simple to implement and can be used to protect against corrosion both in production and in the repair of a wide range of offshore facilities (ships, moorings, slipways, etc.).

List of references

[one]. RF patent №2113544, priority from 04.01.1995, cl. C23F 13/00, IPC ⁶ "Comprehensive protection against corrosion and fouling (options)."

[2]. Andreev N.T. Ship repair. / N.T. Andreev, O.A.Borchevsky, V.G. Lugovykh et al., L .: Shipbuilding, 1972.- 568 p.

[3]. Gavrilkin V.G. The results of measurements of specific electrolytic conductivity and practical salinity of sea water in the international project ccqm-p111. / V.G. Gavrilkin, S.N. Nagibin, A.A. Manskaya et al., Kyiv: State Enterprise "Ukrmetrteststandart": solution@ukrcsm.kiev.ua.

[four]. Kuhling X. Handbook of Physics. Per. with it., Moscow: Mir, 1982, p. 475 (Pl. 39).

Claims (3)

1. A method of corrosion protection of a welded metal structure, including applying an anticorrosive coating made in the form of a continuous conductive layer, characterized in that the conductive layer is applied along the weld and is made of a highly conductive material with a width that is determined by the ratio N _PKP = B + 2L _HAZ + δ where B - width of the weld, L _PEP - HAZ width from one side of the weld, δ - thickness of the welded metal parts, and before application of the electrically conductive layer is conducted zachis ku workpiece surface from the back side of the weld. 2. The method according to claim 1, characterized in that the continuous conductive layer is applied by spraying. 3. The method according to claim 1, characterized in that the continuous conductive layer is obtained from copper.

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