US 2016/0251602 Al  4  Sep.  1, 2016  [0097]  The cleaning liquor can comprise a cleaning formu› lation, which is typically a liquid phase, and the solid particu› late material which can optionally be suspended in, or dis› persed throughout, the cleaning formulation. In certain  embodiments, the density of solid particulate material in the  cleaning liquor (that is, the number of solid particles per unit  volume of cleaning liquor) can be such that any given solid  particle is in frequent, or substantially continuous, contact  with the adjacent solid particles. Thus, in some embodiments,  the cleaning liquor can be densely populated with the solid  particulate material such that it is in the form of a slurry.  rial from the surface of the metal substrate. "Cleaning" of the  metal substrate does not contemplate removal of material  which is integral with the metal substrate surface, such as by  being chemically bound to the metal of the metal substrate.  For example, the removal or partial removal of an oxide layer  formed at the surface of the metal substrate is not contem› plated by the term "cleaning" of the metal substrate.  [0105]  In some embodiments, the cleaning formulation can  comprise at least one surfactant. The cleaning formulation  can thus include one or more surfactants selected from non› ionic surfactants, anionic surfactants, cationic surfactants,  ampholytic and/or zwitterionic surfactants, and semi-polar  nonionic surfactants.  It  is believed that the presence of a  surfactant in the cleaning formulation can facilitate an inter› action with the surface of the metal substrate which can  enhance the cleaning effect of the treatment. The surfactant  can also reduce the surface tension of the cleaning formula› tion allowing better contact between the solid particles, the  cleaning formulation and the metal substrate. The surfactant  may also help to suspend small particles of surface contami› nants which are removed from the surface of the metal sub› strate.  [0098]  In further embodiments, a stream of cleaning liquor  can be directed at the surface of the metal substrate. The  method of the invention can therefore include the use of  spraying apparatus such as pressurized nozzles or the like to  direct the treatment liquor at the metal substrate surface.  [0099]  In other embodiments, the metal substrate can be  moved so that its surface is brought into contact with the solid  particulate material. Such an interaction can be achieved by  rotating or oscillating the metal substrate when suspended by  a holding device at an appropriate position within a portion of  the cleaning liquor containing the solid particulate material.  [0100]  In examples, the formulation comprising the solid  particulate material can be contained within a suitably sized  treatment vessel or chamber. The metal substrate can be  attached to a moveable arm or gripping device which is con› figured for rotation and/or oscillation and/or reciprocation.  The speed, rate or extent of rotation and/or oscillation and/or  reciprocation can be varied to increase or decrease the degree  of mechanical interaction between the metal substrate surface  and the solid particulate material.  [0106]  In some embodiments of the invention, the cleaning  formulation can comprise a non-ionic surfactant. Examples  of suitable non-ionic surfactants include, but are not limited  to, Mulan 200Sfi, alcohol ethoxylates (e.g. C$_{14}$ _ $_{15 }$alcohol 7  mole ethoxylate (Neodol 45-7)), polyoxyethylene glycol  alkyl ethers (e.g. Brijfi, octaethylene glycol monododecyl  ether and pentaethylene glycol monododecyl ether), polyox› ypropylene glycol alkyl ethers, glucoside alkyl ethers (e.g.  decyl glucoside, lauryl glucoside, octyl glucoside), polyoxy› ethylene glycol octylphenol ethers, polyoxyethylene glycol  alkylphenol ethers, glycerol alkyl esters (e.g. glycerol lau› rate), polyoxyethylene glycol sorbitan alkyl esters ( e.g.  polysorbate), sorbitan alkyl esters (e.g. spans), cocamide  MEA, cocamide DEA, dodecyldimethylamine oxide, block  copolymers of polyethylene glycol and polypropylene glycol  (e.g. poloxamers), polyethoxylated tallow amine (POEA).  [0101]  In preferred embodiments the cleaning liquor con› tacts the metal surface at a relative velocity of at least 1 emfs,  more preferably at least 10 emfs, even more preferably at least  50 emfs and especially at least 100 emfs. Preferably, the  cleaning liquor contacts the metal surface at a relative veloc› ity of no more than 100 mis, more preferably no more than 50  mis and especially no more than 10 mis per second. In some  embodiments it is preferred that the solid particles contact the  metal substrate at a frequency of at least 1, more preferably at  least 10, even more preferably at least 100 and especially at  least 1000 particles per second per cm $^{2 }$of surface of the metal  substrate. In some embodiments it is preferred that the solid  particles contact the metal substrate at a frequency of no more  than 1,000,000, more preferably no more than 100,000 and  especially no more than 10,000 particles per second per cm 2  of surface of the metal substrate.  [0107]  In some embodiments of the invention, the cleaning  formulation can comprise a compound with at least one car› boxylic acid moiety. In further embodiments, the cleaning  formulation can comprise a compound with two or more  carboxylic acid moieties, and, in some embodiments, at least  three carboxylic acid moieties. In certain embodiments, the  cleaning formulation can comprise at least one citrate moiety  and can include, for example, citrate containing salts such as  sodium citrate and trisodium citrate.  [0108]  In further embodiments, the cleaning formulation  can comprise one or more metal chelating agents. Examples  of suitable chelating agents can include, but are not limited,  citrates such as trisodium citrate and sodium citrate, phos› phonates ( e.g. Nitrilotrimethylenetris(phosphonic acid), eth› ylenediamine tetraacetic acid (EDTA), gluconates ( e.g.  sodium gluconate) and oxalate. In some embodiments the  inclusion of one or more metal chelating agents in the clean› ing formulation is believed to promote a surface interaction  with the metal substrate which can facilitate the removal of  unwanted materials from the substrate surface.  [0109]  In certain embodiments of the invention the clean› ing formulation can comprise at least one acid. In some  embodiments, the cleaning formulation can include acids  selected from, but not limited to, carboxylic acids such as  citric acid, gluconic acid, adipic acid, acetic acid, lactic acid,  glycolic acid, oxalic acid and formic acid, polycarboxylates  [0102]  Alternatively or in addition, the solid particulate  material can itself be stimulated to move such that the solid  particles are constantly in motion within the cleaning liquor.  In one suitable construction, the method can utilize an agitat› ing device such as an aerator to bubble gas through the clean› ing liquor at a rate sufficient to agitate the solid particulate  material.  [0103]  In certain embodiments, at least some of, and, in  some further embodiments, substantially all of, the solid par› ticles can be buoyant with respect to the cleaning liquor or  formulation. Buoyant particles can be particularly suitable in  embodiments in which an agitating device such as an aerator  is used to bubble gas through the cleaning liquor at a rate  sufficient to agitate the solid particulate material.  [0104]  It  should be noted that the term "cleaning" in rela› tion to the metal substrate and in the context of the present  disclosure contemplates the removal of contaminating mate-