GB2189261A - Processes and compositions for abrasive blast cleaning - Google Patents

Description

GB 2 189 261 A 1

SPECIFICATION increase the reactivity.

Processes and Compositions for Abrasive Blast The solution should therefore be free of Cleaning components that will accelerate the activity of the It is common to use abrasive blast cleaning, i.e., solution and the solution is generally also "shot-blasting" to clean and prepare large areas of, 70 substantially free of metal or ammonium or amine especially, hot rolled steel, as a preparation for cations that could enter into the coating.

painting. It is becoming increasingly common, for The solution is therefore best provided by health and safety and environmental reasons, to dissolving in water either an imine phosphate (often inject water into the airlabrasive stream, thus in combinatin with free phosphoric acid) or free suppressing dust and fumes which are otherwise 75 phosphoric acid and a water soluble imine dangerous and a nuisance to the operator and the compound, generally an imine carbonate.

surroundings. The resultant process may be termed The imine compound contains a group C=NH.

"wet blasting---. An advantage of wet blasting is that Any such compound that is soluble in phosphoric it appears to remove soluble corrosive salts that are acid may be used. Suitable imine compounds not removed by dry blasting. However wet blasting 80 include amino guanidine, auramine, creatine, suffers from a major disadvantage which is that the dicyandiamidine, guanidine, phthalimide, pyrrol, freshly blasted steel surface is very clean and pyrrolidine, pyrroline, rosinduline and B reactive but is damp and so rusts freely. The triphenylguanidine butthe preferred imine resultant rust bloom on the surface is regarded as compound is guanidine, especially because of its detrimental to the properties of subsequently 85 availability as a carbonate which readily dissolves in applied paint coatings. free phosphoric acid. Thus the preferred It is known to include an inhibitor in the injected compositions of the invention are made by blending water with the intention of preventing this rusting. guanidinium carbonate ([NH2)2C=NH]2H2C03)with The inhibitors that have been proposed phosphoric acid in water, generally deionised water.

commercially are sodium nitrite and a 90 The ratio by weight phosphoric acid (measured as polyphosphate such as material sold under the 100%) to guanidinium carbonate is preferably from trade name Nalfloc 918. These are both very 3 to 0.3A, most preferably 0. 5 to 0.75: 1. Equivalent sensitive to concentration problems. Thus sodium amounts of other guanidine or other imine nitrite can reduce the rate of rust formation if compounds can be used. The amounts are not very applied at the right concentration but if it is applied 95 critical. If the amount of phosphoric acid is higher at a concentration that is too low or too high it can than the optimum most of it will still react accelerate rust formation. Polyphosphate permits satisfactorily with the iron surface and any that does rust formation if it is applied at a concentration that not react will generally be rinsed from the surface is too low and if it is applied at a concentration that during application, especially if the surface is given is too high, or if the wash water accumulates and 100 a post rinse. If the amount of quanidinium dries on the surface, the adhesion of subsequently compound is above the optimum then the applied paint is unsatisfactory. The films are water composition may not be a true solution, in that soluble and are liable to be rinsed off the surface. some of the guanidinium compound may remain It is well known to form phosphate coatings, on out of solution. Provided the amount is nottoo high ferrous surfaces by application to the surface of an 105 this may be technically acceptable even though is is acidic phosphate solution optionally containing economically undesirable.

heavy metal ions. In order to obtain a protective The concentration of guanidine (measured as coating it is necessary to include nitrate or other guanidinium carbonate) is preferably in the range accelerator system. However such solutions are not 0.2 to 4.5 911 with best results being achieved at satisfactory for inclusion in the blast water during 110 around 1.5 to 2. 5 g/L If the solution is too weak the wet blasting as either they do not prevent rusting or rust inhibition is inadequate and if the solution is too they form a coating which does not provide strong it may have detrimental effects on the satisfactory adhesion of subsequently applied paint. subsequently applied paint films.

It would be desirable to be able to prevent rusting It appears that the treatment according to the during wet blasting and to promote good adhesion 115 invention forms a chemically bonded coating of subsequently applied paint, and in particular it containing iron, imine and phosphate.

would be desirable to provide such a system The contact with the iminephosphate solution is wherein satisfactory results can be achieved over a conducted throughout the wet blasting or merely at range of concentrations and irrespective of whether the end of the wet blasting, by using the solution as or not the coated surface is rinsed with water after 120 the water that is injected into the airlabrasive blast.

the treatment. The contact of the treatment solution with the iron In the invention an iron surface is cleaned by wet surface during wet blasting is generally for from 0.1 blasting and rusting of the cleaned surface is to 2 minutes. Contact is generally effected at prevented or reduced by contacting the surface temperatures below WC, generally 10 to 2WC during blasting with an aqueous solution containing 125 solution temperature.

phosphate ions and, as an inhibitor, an imine The wet blasting may be conducted in compound. conventional manner. Suitable abrasives include It is normally necessary to activate phosphate steel shot and grit, flint shot, mineral slag or any solutions so as to improve their reactivity with the other acceptable abrasive medium for use on steel coating but in the invention we inhibit rather than 130 by wet-blasting methods. Pressure of the air/ 2 GB 2 189 261 A 2 abrasive blast is generally in the range 3.5 to 10 kg/ 3. A process according to claim 1 in which the cm'. The amount of water that is injected is imine compound is guanidine.

generally in the range 0.5 to 5 litres per minute. 4. A process according to claim 1 in which the The following is an example. imine compound is guanidine introduced as salt Hot rolled steel is subjected to wet blasting by a 35 selected from guanidine carbonate and guanidine stream of air at 7 kg/cm' in which is entrained phosphate.

mineral slag abrasive and a solution of 2 g/1 5. A process according to claim 1 in which the guanidinium carbonate and 1.32 911 phosphoric acid phosphate ions are introduced as phosphoric acid.

(100%) is injected into the stream during wet 6. A process according to claim 1 in which the said blasting. 40 aqueous solution contains guanidine present in an After contacting the surface with the imine amount of 0.2 to 4.5 g/I measured as guanidine solution the surface is allowed to dry, optionally carbonate and the ratio of phosphate ions after rinsing with water. Conventional paint may (measured at 100% phosphoric acid) to guanidine then be applied over the surface. There is (measured as guanidine carbonate) is 0.3:1 to 3: 1.

substantially no rusting of the surface before 45 7. A process according to claim 1 in which the said painting and the paint adheres well. aqueous solution includes guanidine and the amount of guanidine is 1.5 to 2.5 g/I measured as'

Claims (2)

CLAIMS guanidine carbonate and the ratio of phosphate

1. A process in which an iron surface is cleaned by (measured as 100% phosphoric acid) to guanidine, wet blasting using a stream of air, water and 50 measured as guanidine carbonate, is 0.5 to 0.75: 1.

abrasive particles and in which rusting of the 8. A process according to claim 1 in which the said cleaned surface is reduced by contacting the surface aqueous solution is used as the water in the wet during blasting with an aqueous solution containing blastfor the final 0. 1 to 2 minutes at a temperature phosphate ions and, as an inhibitor, an imine below WC.

compound. 55 9. A solution of 0.2 to 4.5 g/1 guanidine (measured

2. A process according to claim 1 in which the as guanidine carbonate) and phosphate ions imine compound is selected from amino guanidine, present in a ratio of phosphate ions (measured as auramine, creatine, dicyandiamidine, guanidine, 100% phosphoric acid) to guanidine (measured as phthalimide, pyrrol, pyrrolidine, pyrroline, guanidine carbonate) of 0.3 to 3: 1.

rosinduline and 13-triphenylguanidine. 60 Printed for Her Majesty's Stationery Office by Courier Press, Leamington Spa, 10187. Demand No. 8991685. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.