Coating processes – Interior of hollow article coating
Patent
1989-02-06
1991-09-24
Bell, Janyce
Coating processes
Interior of hollow article coating
118408, 118410, 118602, 427235, 427239, 427345, 4274301, 427435, B05D 722, B05C 1100
Patent
active
050512811
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process and a plant which enable the corrosion of the inside walls, made of ferrous metals or based on cement, of pipeworks for transporting water to be avoided.
Water transported in pipe networks may be in contact with ferrous metals (cast iron, steel, galvanized steel) or materials based on cement (coatings of cement mortar on metallic pipeworks, reinforced concrete, asbestos cement). In order to simultaneously avoid the corrosion of ferrous metals and that of materials based on cement, it is essential that this water should be capable of giving rise to a protective precipitate of calcium carbonate (CaCO.sub.3). In other words, it must be encrusting.
On the graph of total CO.sub.2 --Ca.sup.2+ (cf. Les equilibres carboniques et l'equilibre calcocarbonique dans les eaux naturelles, Eyrolles 1981, by the present applicants), the graphical point for the composition of a water of this kind must correspond to the conditions below (see FIGS. 1 and 2):
It must be located to the right of the calcium/ carbonate equilibrium curve (curve C). Indeed, waters plotted to the left of this curve are aggressive with respect to solid CaCO.sub.3.
It must be located to the left of the curve of spontaneous precipitation of CaCO.sub.3 (curve C'). Indeed, waters plotted to the right of this curve, which are unstable, are observed to form and develop within their bulk nuclei of CaCO.sub.3 not being able to give rise to an adhering precipitate on the walls of the pipeworks.
It must be slightly to the right of the curve C. If it is not so, the thickness of the CaCO.sub.3 precipitate formed on the walls of the pipeworks could be too great.
It must be located above the semi-straight line SX, of slope 2. If not, on the one hand the formation of the precipitate of solid CaCO.sub.3 could be at risk, and on the other hand the sterilization of the transported water by chlorine would not be guaranteed, the pH of the water being too high. The abscissa of the point S of the semi-straight line SX depends on the concentration of the ions.
It is known that waters which are available are often aggressive towards solid CaCO.sub.3. It is possible to make them encrusting by adding lime (Ca(OH).sub.2), which has to be preceded by, if appropriate, by an addition of carbon dioxide (CO.sub.2).
As a result of the addition of Ca(OH).sub.2, the point on the graph for the water moves from M.sub.1 located to the left of the curve C to M.sub.2 located slightly to the right of the curve C (see FIGS. 3, 4, 5 and 6). At a preliminary stage, it moves from M.sub.0 to M.sub.1 as a result of the addition of CO.sub.2, which is necessary when addition of Ca(OH).sub.2 alone does not enable the treated water to take up a plotted position above the semi-straight line SX, of slope 2 (see FIGS. 5 and 6).
Lime is supplied by the chemical industry in powder form. The use of lime in this solid form is only possible under the two following conditions:
The water must undergo a clarifying treatment before being put into the network, this treatment comprising, in succession, flocculation, possibly decantation, finally filtration. The lime is then added to the water before flocculation;
The coagulant used must not be an aluminium salt. Indeed, making the water slightly enriched in calcium by adding lime results in an increase in the pH. If the coagulant used were an aluminium salt, there would be the risk that the water coming out of the treatment station would have a concentration of Al.sup.3+ ions greater than the maximum concentration set by health standards.
If these conditions are not met, the lime must be added to the water in the form of a solution, called lime water, which is produced in a lime saturator. However, for reasons which will be explained later on, the use of a lime saturator of the current type has two disadvantages:
the large volume of the device;
and particularly the impossibility of giving rise to a solution with a well defined lime content; now, adding Ca(OH).sub.2 to an aggressive water only guarantees the formation of a
REFERENCES:
patent: 1683521 (1928-09-01), Bourgognion et al.
patent: 3640759 (1972-02-01), Primus et al.
patent: 4264651 (1981-04-01), Hasson et al.
patent: 4678685 (1987-07-01), Hasson et al.
"Corrosion Prevention by Controlled Calcium Carbonate Scale", Industrial and Engineering Chemistry, vol. 37, No. 9, Sep. 1945, by S. Powell et al., pp. 842-846.
Legrand Luc
Leroy Pierre
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