Abstract:
A process for the protection of stainless surface from corrosion by a hypohalite-containing composition, the process comprising:-  
     (a) contacting the surface with a composition comprising hydrogen peroxide; and  
     (b) contacting the surface with the hypohalite-containing composition.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a process for improving the corrosion resistance of stainless steel.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the production or storage of hypochlorite-based products, stainless steel vessels exposed to those are rapidly subject to corrosion.  
           [0003]    EP-A-096 151 describes a method for passivating the surface of strippers of urea manufacturing plants, using a first passivating agent consisting of an oxygen-containing gas and a second passivating agent in liquid form, selected from hydrogen peroxide, alkali metal persulphates, alkali metal perborates, peracetic acid and organic peroxides.  
           [0004]    A method for passivating stainless steel using sulphuric acid, Fe 3+ , hydrogen fluoride, hydrogen peroxide, non-ionic surfactants and acid attack inhibitors is disclosed in U.S. Pat. No. 5,354,383.  
           [0005]    A method of passivating a stainless steel article having a surface oxide layer to be passivated, using a dry gaseous fluid chemically non-reactive with the stainless steel, flushing the surface and baking the article, is described in U.S. Pat. No. 5,188,714 the gas is typically nitrogen or argon. Manganese dioxide, e.g. precipitated from a potassium permaganate solution, to coat the walls of a polymerization reactor, is known from EP-A-327 202.  
           [0006]    FR-A-625 397 discloses a process of passivating a chrome-nickel steel reactor for urea synthesis, using a small quantity of an oxygenating substance such as a peroxide.  
           [0007]    We have now found that a hydrogen peroxide solution is a simple and effective medium for passivating stainless steel subsequently to be exposed to a hypohalite-containing medium.  
         DEFINITION OF THE INVENTION  
         [0008]    Thus, the present invention now provides a process for the protection of stainless surface from corrosion by a hypohalite-containing composition, the process comprising:  
           [0009]    (a) contacting the surface with a composition comprising hydrogen peroxide; and  
           [0010]    (b) contacting the surface with the hypohalite-containing composition.  
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    The present invention protects stainless steel surfaces of pipes, vessels and the like for the pumping, transporting, storage or preparation of hypohalite-containing compositions or of any other stainless steel surface to come into contact with the hypohalite composition.  
         [0012]    Preferably, the hypohalite is hypobromite or hypochlorite, especially hypochlorite.  
         [0013]    The hydrogen peroxide may be provided as a simple aqueous solution or in a (preferably aqueous) composition, optionally containing one or more other ingredients, such as surfactants. A surfactant system in an aqueous medium is one suitable composition.  
         [0014]    The hydrogen peroxide composition preferably contains from 0.1% to 10%, more preferably from 1% to 5% by weight of hydrogen peroxide.  
         [0015]    A typical aqueous medium containing a surfactant system, comprises from 0.01% to 1%, preferably from 0.1% to 1% by weight of anionic surfactant and from 0.1% to 10%, preferably from 1% to 5% by weight of nonionic surfactant.  
         [0016]    Preferably, the hydrogen peroxide containing medium also contains a hydrogen peroxide stabiliser such as a heavy metal sequestrant, e.g. of the phosphonate type.  
         [0017]    Typical hypohalite-based products against which the stainless steel is to be protected include aqueous hypochlorite bleaches, especially aqueous thickened alkali metal hypochlorite bleaches.  
         [0018]    It has also been found that passivation of this stainless steel surface is synergised if the hypohalite composition comprises an alkali metal silicate, preferably sodium silicate.  
         [0019]    The concentration of the alkali metal silicate in the hypohalite composition in which it is applied is preferably from 0.01% to 5%, more preferably from 0.1% to 1% by weight of that composition. optionally, the hypohalite composition may contain from 1% to 25%, e.g. from 10% to 20% by weight of an amine oxide surfactant.  
         [0020]    The surface may be treated with the hydrogen peroxide containing medium by filling of a container, the inside surface of which is to be passivated, or by spraying the surface to be subsequently exposed to the hypochlorite, or by any other means. Preferably it is removed, e.g. by drainage, before the surface is contacted with the hypohalite composition.  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    A 40 litre stainless steel vessel was filled with a hydrogen peroxide containing pre-treatment composition and left for an hour. The peroxide-containing composition was then drained and the vessel was filled with a thickened bleach composition.  
         [0022]    The compositions were as follows:  
                                                                                   wt %                    Pretreatment   Bleach                            Water   76.42   45.38           Empigen OC/E (1)   —   17.39           Ethomeen BTB12 (2)   3.00   —           Eltesol SCS40 (3)   1.37   —           Prifac 2920 (Lauric Acid)   —   2.69           Lial III-10EO Nonionic   0.13   —           Sulphamic Acid   4.00   —           Sodium Silicate   —   0.53           Sodium Hydroxide   —   1.33           Sodium Hypochlorite (15%)   —   32.24           Hydrogen Peroxide (30%)   14.29   —           Phosphonate Sequestrant   0.45   —           Colour, Perfume   0.34   0.44                                                          
 
         [0023]    The vessel was inspected daily for evidence of corrosion. On a weekly basis, the bleach was drained and the available chlorine measured, to check that no decomposition (and associated loss of corrosivity) had occurred. The vessel was then re-filled with product. No further peroxide treatment was undertaken after the initial pre-passivation.  
         [0024]    No corrosion was observed after a period of 4 weeks, except on the rim of the vessel, under the outside of the gasket. Here, crevice corrosion occurred, but this region was not wetted during the peroxide pre-passivation.