Abstract:
A multilayer pipe surface coating for water system elements requiring high corrosion resistance includes at least one conversion layer and one thermosetting synthetic resin layer.

Description:
BACKGROUND OF THE INVENTION 
     This invention concerns a multilayer anti-corrosive coating for the entire surface of a ferrous metal element, such as a water system pipe. 
     Petroleum bitumen-based coatings are known and used as internal and external coatings for water system elements. These coatings are low-cost but require the use of solvents, which increases the risk of fire. 
     In other known coatings, all the layers are obtained from epoxy powder-based compositions, which offer good corrosion resistance. But the thickness of such coatings varies, especially at the joints where the elements are dimensioned to fit together for assembly (seal grooves, centering diameters), thereby making it necessary either to increase the number of foundry models with different dimensions for the different coatings, or to machine down the metal elements prior to coating. 
     These coatings consist of inert material forming a barrier against external elements. This barrier, however thick it may be, is still subject to impact damage that will bare the substrate, at which point protection no longer exists, without involving any chemical reactions between the coating and its substrate. This is true of both petroleum bitumen and epoxy powder. In order for this protection to have maximum effectiveness, it is important that the bitumen or epoxy be free of damage down to the metal. 
     If these conditions are not met, corrosion sets up electrical macrocells due to damage from transverse impacts, cracks, and separations which, by locally depassivating the iron by lowering the pH and allowing depassivating substances to enter, will localize isolated anodic sites within the cathodic areas, which are the passivated surfaces where the coating still remains in intimate contact with the metallic structure. From that point on, as the ratio of the cathodic surface area to the anodic surface area is generally large, the current flowing in these macrocells may be high whenever any active depolarization of the cathodic area is possible. 
     SUMMARY OF THE INVENTION 
     The present invention aims to remedy these disadvantages and to provide a low-cost coating covering the entire internal and external surface of a metal water system element, with an automated application process. 
     In the invention, the metal element with its coating must exhibit good resistance to corrosion from acid or basic soils such as, for example, calcareous, clayey soils or soils bearing sweet or salt water tables. The coated element must also stand up to corrosive attack by the different kinds of water that may be transported by the element such as, for example, drinking water, treated water, wastewater or rainwater. The coated element must not give off toxic products, or any that will discolor the water or alter its taste or smell. 
     To achieve low manufacturing cost, the coating must follow the functional form of the metal element, such as its grooves and flanges, without requiring touchup after the coating is applied. The coating should allow material to be saved, by avoiding surplus application such as by running and excess thickness, and the coated element must also stand up to corrosion by the air, to which it is subject during storage and shipment, prior to installation. Air corrosion is linked to variations in the temperature and humidity of the atmosphere. This results in a general attack on the entire surface of a metal element, unlike corrosion by the soil, which consists, among other things, of localized pile effects as explained above. 
     Finally, the element according to the invention must not exhibit any accumulation of corrosion macrocells and must offer good impact resistance to the damage that gives rise to the macrocells. 
     To do so, the multilayer coating for the entire surface of a ferrous metal water system element includes, building from the metal element outward, at least one conversion coat and one thermosetting synthetic resin coat of constant thickness. In one variant, the conversion coat is passivated. 
     Furthermore, it has been discovered that such a coating exhibits very effective corrosion resistance if the conversion layer of the coating consists of a combined zinc and iron phosphate. 
     In accordance with other characteristics or variants of the coating: 
     the passivation is chromic or organic, 
     the organic products of the passivation include tannic acid, 
     the thermosetting synthetic resin is an epoxy, and 
     it includes an additional layer of thermosetting synthetic resin covering the first layer thereof. 
     The invention also relates to a process for applying a coating as described above over the entire surface of a ferrous metal element. The process includes at least the following phases, in succession: 
     a chemical conversion bath, 
     rinsing with demineralized water, 
     electrodeposition in a bath including a synthetic resin in an aqueous medium, and 
     curing of the synthetic resin. 
     Other variants and characteristics of the process exist, with the following differences: 
     Between the rinsing with demineralized water and the electrodeposition, a passivation bath is added, followed by another rinsing with demineralized water. 
     The electrodeposition bath liquid can be enriched in synthetic resin by ultrafiltration. 
     An extra outer coat of synthetic resin can be added to the metal element. 
     This additional coat of synthetic resin is applied by electrostatic dusting, on cold or hot metal elements, by electrostatic spray, by immersion in a fluidized bath, by electrophoresis followed by a curing, or by spraying the two components of a liquid synthetic resin. 
     Another object of the invention is the application of the coating to the entire surface of a water system pipe or like element, consisting of a carbon alloy of a ferrous base metal, the metal element being of irregular shape, capable of transporting water, and being in contact with soils. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation drawing with partial cutaway of a pipe element with a multilayer coating in accordance with the invention, and 
     FIG. 2 is a histogram of test results showing the consumption, over time, of a coating according to the invention, compared with that of other coatings on the entire surface of a water system element. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A rough-cast ferrous metal element 1, illustrated as a pipe system elbow in FIG. 1, is installed in earth or soil 2. It is covered with a multilayer coating 5 including, in contact with the metal element, a conversion layer 3 covered with a layer of thermosetting synthetic resin 4. The thickness of each of the layers is constant and follows the original form or shape of the element 1, which is thus preserved at the surface of the coating 5. The shape of the element 1 is complex, as it has a bend and is connected to other water system piping elements, which are not illustrated. Each joint with the other water system elements is a jam fit, 6a or 6b. Each of these joints 6a or 6b (as disclosed, ex., in commonly assigned French patent application FR-A-2,647,520) has a coaxial groove at its inlet end, followed by a flange. The groove consists of a slightly tapered conical face that continues to the bottom of the joint 6a or 6b, or a cylindrical surface of constant diameter, terminated in the internal radial direction by a flat or slightly tapered surface, perpendicular to the line X--X or X&#39;--X&#39;. This flat surface is extended toward the bottom of joint 6a or 6b by a convex curve and by a truncated conical surface that fits at last with a cylindrical surface of a diameter less than that of the cylindrical surface of the groove of joint 6a or 6b. The tapered surface is conical outward toward the bottom of the joint, such that the inner surface of the joint 6a or 6b can be fitted with a packing seal. The cylindrical surface ends at the bottom of the joint with a radial stop that limits the length of the cylindrical surface. 
     The metal element i is produced with the desired configuration of its end joints by foundry casting, and the coating 5 is then applied to the entire surface of the ferrous metal element, i.e. its external and internal faces 7 and 8, respectively. This application is performed by subjecting the element to a chemical conversion bath, followed by rinsing in demineralized water, a passivation bath, rinsing in demineralized water, and finally by electrodeposition in a bath including a synthetic resin in an aqueous medium. 
     The electrodeposition bath is further rinsed by ultrafiltration. 
     Considering that the particular shape of element 1 has been maintained, it is not necessary to perform any touchup operations on the element after it is coated. 
     The purpose of the invention is to: 
     apply a coating 5 in conformity with the one described above, covering the entire internal and external surfaces of a cast element for transporting drinking water, and/or 
     apply a coating 5 in conformity with the one described above, to cast iron parts. 
     FIG. 2 shows a histogram of test results on different coatings: one coating 9 of petroleum bitumen base; and a coating 10 obtained from an epoxy powder base compound. These known coatings 9 and 10 are compared with a coating 5 according to the invention, in which the conversion coat consists of a combined zinc and iron phosphate. 
     The tests consisted of simulating damage passing through the coatings 5, 9, 10 in a medium of low resistivity (100Ω×cm), creating an electrochemical couple constituting a macrocell as described above. This simulation was performed on a sample cut out of a coated metal element. 
     A cylindrical anode of the same kind as the metal element was placed inside the sample. The anode was encased in a sheath of electrical insulation from which only one of the end disks protruded. The anode was connected to the sample by an electrical connection including a microammeter. 
     The objective was to compare how much anode metal is consumed after 200 days in a medium having a resistivity of 100Ω×cm, for the three types of coatings. The metal consumed is represented in the histogram ordinate in millimeters per year. 
     These tests in the medium of 100 Ω×cm resistivity are also presented in the following table which compares the coating according to the invention with a bitumen-base coating, over intervals of 100, 158 and 200 days, and with an epoxy-base coating applied by dusting on cold parts, for an interval of 200 days. The anode metal consumption was calculated as a function of the current flow (level of attack). 
     The results are brought together in Table 1 below. 
     
                       TABLE 1______________________________________   Medium of 100 Ω × cm resistivity     Current  Current  CurrentReference (μA)  (μA)  (μA)                              Level of attack______________________________________Bitumen-base     83       72       60     3.9coatingEpoxy coating               15.4   1.0dusted on coldpiecesCoating   5.6      7.3      8.3    0.44according to theinvention______________________________________ 
    
     These three coatings were also compared in a less aggressive environment, over intervals of 100, 158 and 200 days, and the anode metal consumed was calculated as a function of the current flow (level of attack). 
     The results are summarized in Table 2 below. 
     
                       TABLE 2______________________________________   Medium of 1500 Ω × cm resistivity     Current  Current  Current     (μA)  (μA)  (μA)                              Level of attack     at 100   at 158   at 200 (mm/year)Reference days     days     days   (base 200 days)______________________________________Bitumen-base     70       53       43     2.8coatingEpoxy coating               9.8    0.64dusted on coldpiecesCoating   2.8      2.8      3.4    0.22according to theinvention______________________________________ 
    
     Measurements were also made of the variation of the corrosion current, as determined from Tafel graphs (in μA/cm 2 ) for the coating according to the invention and another coating without an underlying chemical conversion layer. The tests lasted one to four days, 30 days and 145 days. 
     The results are given in Table 3 below. 
     
                       TABLE 3______________________________________      1 to 4 days                 30 days 145 days______________________________________Thermosetting        170          135     430resin-basecoating withoutchemicalconversion layerCoating according         28           14      90to the invention______________________________________