METALLIC SURFACE PROTECTION

Abstract: A process of treating corrosive liquids to protect metallic surfaces exposed to such liquids, comprising the step of increasing the redox potential of the corrosive liquid, for example, by means of hydrogen peroxyde, before it contacts the metallic surface to be protected, whereby the process liquid causes the formation of a protected oxide layer on said metallic surface.

Description: METALLIC SURFACE PROTECTION FIELD OF THE INVENTION: This invention relates to the treatment of chemical process liquids which cause corrosion, and ultimately, stress corrosion cracking, of metallic surfaces exposed to such liquids. BACKGROUND OF THE INVENTION: Ferrous materials, including both plain and alloyed carbon steels, are susceptible to a corrosion phenomena known as stress corrosion cracking (SCC) or caustic embrittlement when exposed to hot caustic liquors. This can cause significant problems in industrial plants utilizing hot caustic liquors for processing. For example, alumina refineries (e.g. Bayer plants) which digest bauxite ores in sodium hydroxide at temperatures up to 300°C use plain carbon pressure vessel steels for pipework and vessels in the liquor stream. Similarly, pulp digesters in the paper industry are exposed to the same type of environment. Plain carbon and alloy steels remain relatively inert under these conditions due to the formation of protective oxides and scales. They are, however, sensitive to stress corrosion cracking, a process involving selective intergranular or transgranular attack which penetrates deep into the metal structure. SCC occurs under a range of specific conditions in the process stream and has been loosely allied to liquor temperature, caustic strength and stress level in the metal. The incidence and intensity of attack increases with increasing levels of the three factors. As the name SCC implies, the stress factor is an important factor in the mechanism of attack. Stresses can arise during fabrication (e.g. along weldments, bends, etc.) or can be introduced due to temperature and/or pressure fluctuations in the process stream. Measures taken to avoid or reduce the risk of SCC include: i) post weld stress relief ii) temperature limits SUBSTITUTE SHEET iii) caustic strength limits iv) anodic protection (paper industry) The specification of temperature and caustic limits reduces the range of operating conditions which can in turn limit plant productivity. Post weld stress relief is regarded as one of the most effective mitigation measures but cannot be controlled sufficiently to ensure full protection against SCC failures. Considerable effort has also been directed towards developing cost-effective SCC resistant alloys but further work is required before this can be achieved. BRIEF DESCRIPTION OF THE INVENTION AND OBJECTS: It is an object of the present invention to provide a process of treating corrosive liquids to protect metallic surfaces exposed to such corrosive liquids against corrosion to thereby materially reduce the incidence of stress corrosion cracking. The invention provides a process of treating corrosive liquids to protect metallic surfaces, and particularly ferrous metallic surfaces, exposed to such corrosive liquids, comprising the step of increasing the redox potential of the corrosive liquid before it contacts the metallic surface to be protected whereby the process liquid causes the formation of a protective oxide layer on the metallic surface. The invention may be applied to any corrosive liquid and is particularly applicable to hot caustic liquids. The process of the invention results in the metallic surfaces ^contacted by the treated caustic liquid being passivated by the formation of the protective oxide layer, which in turn substantially alleviates the problem of stress corrosion cracking. The invention is specifically applicable to the treatment of Bayer process liquors to reduce the incidence of stress corrosion cracking of plant materials contacted by the process liquors. However, the invention is equally applicable to other corrosive liquors in other industries, such as the paper industry. The step of increasing the redox potential of the corrosive liquid may be achieved by the addition of any suitable oxidizing agent to the corrosive liquid. Suitable oxidizing agents include H2O2, NaH0 2 , O2, O3, NaN02 and Mnθ4» Such oxidizing agents increase the redox potential of the liquid which in turn promotes passivity of the metallic surface. Brief Description of the Drawings: A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a graph showing the effect of caustic concentration and temperature on SCC plain carbon steel in Bayer liquor, and Figure 2 is a graph shown the effect of H2O addition SCC sensitivity. Description of Preferred Embodiment: In the Bayer process sodium hydroxide is used to dissolve alumina from the bauxite ore at temperatures up to 300°C. The so-called pregnant liquor is subsequently cooled to 100°C through a series of flash tanks and settled and filtered to remove the insoluble residue. Alumina is then precipitated from the caustic liquor which is reheated in a series of heaters and returned to the digester. Following the precipitation the recycled liquor is referred to as spent-liquor and it is this liquor which contributes predominantly to stress corrosion damage. The components in the spent liquor circuit susceptible to SCC include the heater tubes and tube sheets and the interconnecting pipework between each heater. The latter are particularly sensitive to SCC along weldments. In general welds are stress relieved to reduce the risk of SCC. The incidence and intensity of SCC in this part of the circuit has been found to increase along the heater chain as the temperature of the liquor increases. Treatment of the process liquor is preferably performed at the low temperature side of the spent liquor circuit, but it may also be undertak he- high established for a range of liquor temperatures and caustic concentrations together with the effect of hydrogen peroxide dosing on material exposed to caustic liquors above the threshold level. EXAMPLE 1 : Using the SSRT technique, the effect of liquor concentration and temperature on the stress corrosion sensitivity of a typical pressure vessel steel (Table 1) was established . Table _1_ - Composition of Pressure Vessel Quality Steel Composition wt % C Mn P S Si Fe 0.31 0.85 0.035 0.04 0.05 Rem. to 1.25 The spent liquor used for the tests was taken from the spent liquor circuit of an alumina refinery (Table 2). Table 2. ~ Analysis of Spent Liquor Caustic s Ratio 1. Alumina 84.8 0.378 2. Caustic Soda (as Na2Cθ3) 224.4 - 3. Total Soda 262.9 - 4. Total Sodium 326.8 - 5. P2O5 0.11 0.0005 7. Si0 2 0.47 0.0021 9. V 2 0 5 0.80 0.0036 10, V 2 0 5 as Soda 1.40 - 11 NaCl 14.32 0.0638 12. NaCl as Soda 12.99 - 13, Na 2 S0 4 0.32 0.0014 15, Non Caustic Soda 102.4 0.456 16. Carbonate Soda 38.5 0.172 17. Non Alkaline Soda 63.9 0.285 18. Sodium Oxal ' ate 3.19 0.014 πf υτt sπtet * 19. Oxalate as Soda 2.52 - 20. Total Inorganic Non Alkaline Soda 15 .71 0.070 21. Total Organic Soda 45.67 0.204 22. T00C as Soda 226 .0 1 . 007 23. Fe 2 0 3 0. 004 - 24. C/3 0.854 - 25. S/Total Sodium 0.804 - 26. Kelly Soda 311 . 0 _ The results shown in the graph of Figure 1 clearly identify the temperature and caustic concentration above which SCC can be expected. At normal caustic concentrations (225 g /X expressed as caustic soda) the threshold temperature for SCC is 150°C. At 160°C, SCC can be expected to occur in ferrous components stressed close to or beyond the yield point of the material. Micrographic analysis shows that for SCC non-sensitive regions the fractures under testing are ductile, while SCC sensitive regions the fractures under testing are brittle. The extent of SCC propagation penetrating into the samples from the outer surface was noted to be unacceptable above the threshold temperature. The micrographs also show the marked difference in the reduction of cross-sectional area under the two conditions. EXAMPLE 2: Taking 160°C as a representative temperature at which SCC will dominate the failure mechanism, tensile samples were exposed to liquors containing increasing levels of hydrogen peroxide. The effect of these additions on the reduction in cross-sectional area at the time of failure and, therefore, the mode of failure is shown in the graph of Fig. 2. These results demonstrate the increasing benefit of peroxide additions at 160°C and also show that stress corrosion can be mitigated even at higher caustic concentrations. In the latter case the potency of the peroxide must be increased to reduce the dosing level. This may be improved by adding stabilizers to the peroxide to reduce the rate of decomposition to O2 and H2O.

Claims: CLAIMS : 1. A process of treating corrosive liquids to protect metallic surfaces, and particularly ferrous metallic surfaces, exposed to such corrosive liquids, comprising the step of increasing the redox potential of the corrosive liquid before it contacts the metallic surface to be protected whereby the process liquid causes the formation of a protective oxide layer on the metallic surface. 2. The process of claim 1, wherein the step of increasing the redox potential of the corrosive liquid is achieved by the addition of an oxidizing agent to the liquid. 3. The process of claim 2, wherein the oxidizing agent is selected from H2O21 NaH02ι O2, O3, NaN02 and KMnO^. 4. A process of treating corrosive liquids used in the Bayer process, comprising the step of adding to the liquor used in the process a material which increases the redox potential of the liquor to such an extent that a protective oxide layer is formed on metallic surfaces contacted by said liquor. 5. The process of claim 4 wherein said material is selected from H 2 0 2 , NaH0 , 0 2 , O3, NaN0 2 and KMnO^ 6. The process of claim 4, wherein said material is H2O2 a °d is added to the spent liquor circuit of the process therefore temperature of the spent liquor exceeds about 160°C at a concentration of approximately 0.05 to 1.5% v/v (50 to 3000 of spent liquor. 7. The process of claim 6 wherein said concentration is 200 to 1000 mg/ of spent liquor.

Inventors: MAY ROBIN FRANK, MAY, ROBIN, FRANK
Applicants: COMALCO ALU, COMALCO ALUMINIUM LIMITED
Application year: 1989
Main classification: C23F11
IPC codes: C01D1, C01F7, C23F11