Patent Publication Number: US-2010129544-A1

Title: Polymer-Based Ceramic Coatings for Protecting Surfaces Against Fluoride Ions During a Cleaning Process

Description:
The present invention relates to the gentle cleaning of partially corroded or oxidized surfaces using fluoride ions. The invention also relates to the use of polymer-based ceramic coatings. 
     During operation, the components of a hot-gas passage, in particular the turbine blades or vanes of a gas turbine, are exposed to corrosive hot gases and extreme mechanical stresses. Corrosion, oxidation and cracks therefore regularly appear on the surfaces of these components. Before the cracks are repaired by soldering or welding, it is necessary to clean oxide or corrosion residues away from the affected surfaces and the cracks. This cleaning process is typically carried out using fluoride ions. However, this means that the non-oxidized parts of the surfaces are also attacked by the aggressive fluoride ions. Particularly in the case of nickel-base superalloys, the resultant wear considerably impairs the material properties of the components affected by the fluoride ion cleaning. The wear on the components resulting from the fluoride ions therefore has a disadvantageous effect on the service life of these components. 
     The imminent deterioration of the material means that it is not possible to refurbish cracked components when fluoride ion cleaning has been used beforehand, in particular rotating components which, during operation of a gas turbine, are exposed to particularly high mechanical stresses and therefore have to satisfy stringent requirements in terms of their strength. The result of the material properties of the alloy deteriorating on account of the cleaning process is therefore generally that the corresponding turbine blade or vane has to be replaced completely. The resultant shorter service life of rotating components signifies a considerable economic disadvantage. 
     Oxidation and corrosion are therefore suppressed by coating turbine blades or vanes with a continuous oxidation-resistant thermal barrier coating in order to delay refurbishment. On account of mechanical stresses, however, the material used is unavoidably subjected to the formation of micro-cracks, and as a result oxidation is merely delayed rather than prevented. 
     In order to improve the protection of components made from composite material against oxidation, these components are coated, in EP 0 550 305 B1, with a mixture of a refractory ceramic material and a healing compound. The coating is carried out by applying a polymer precursor and subsequently transforming the precursor into the ceramic. However, even a thermal barrier coating of this type would not eliminate the need for refurbishment using fluoride ions in order to clean turbine components. 
     U.S. Pat. No. 6,645,926 B2 describes a masking system for use during fluoride cleaning. In this system, a multilayered parting component is first of all applied to those parts of the surface which are to be protected. A chromium-containing masking layer is then applied to the parting component. In this case, the masking layer is used to protect the surface against the influence of the fluoride ions. The parting component should make it possible to easily remove the masking layer after the fluoride ion cleaning. 
     The parting component contains colloidal silica, deionized water, fused alumina grains and alumina powder. The masking layer contains chromium powder mixed with a binder, a wetting agent, a thickening agent and water. 
     It is an object of the present invention to provide an alternative process which helps to prevent non-oxidized parts of the surface from becoming worn by fluoride ions. 
     A further object of the invention is to provide an advantageous use of polymer-based ceramic material. 
     In order to achieve the objects mentioned, the present invention provides a process for the gentle cleaning of partially corroded or oxidized surfaces using fluoride ions as claimed in claim  1 , and the use of polymer-based ceramic material as claimed in claim  14 . The dependent claims contain advantageous refinements of the invention. 
     In the process for the gentle cleaning of partially corroded or oxidized surfaces using fluoride ions according to the invention, the non-corroded or non-oxidized parts of the surface to be cleaned using fluoride ions are protected by coating the non-corroded or non-oxidized parts of the surface with a polymer-based ceramic before the cleaning process is started. Polymer-based ceramics (polymer derived ceramics) are a class of materials that can be used at room temperature as conventional polymers (or, if appropriate, as monomers which are subjected to polymerization before ceramicization), in particular for surface coating. A further typical application of these materials is their use as amorphous high-temperature fibers, since these materials remain amorphous up to 1500° C. The polymer-based ceramic affords highly effective protection against attack by fluoride ions. It can also be applied, in a simple and defined manner, in the form of a polymer precursor to the surface regions to be protected. 
     The surfaces to be protected can be coated by applying a monomeric or polymeric precursor of the polymer-based ceramic to the surface and then ceramicizing the precursor. Within the context of fluoride ion cleaning, suitable precursors are primarily those which contain silicon and particularly those which form SiC and Si 3 N 4 . 
     The precursor may contain nitrogen. If the precursor does not contain any nitrogen, the subsequent ceramicization may be brought about by pyrolysis in nitrogen. 
     In particular, the following precursors are suitable for carrying out the coating: polysilane [amorphous phase: Si—C—(O), crystalline phase: SiC and Si 3 N 4  (pyrolysis in N 2 ), C], polycarbosilane [amorphous phase: Si—C—(O), crystalline phase: SiC and Si 3 N 4  (pyrolysis in N 2 ), C], polysilazane [amorphous phase: Si—C—N, crystalline phase: SiC and Si 3 N 4 ], polyborosilazane [amorphous phase: Si—B—C—N, crystalline phase: SiC and Si 3 N 4 ] or polycarbosilazane [amorphous phase: Si—C—N—(O), crystalline phase: SiC and Si 3 N 4 ]. The use of polycarbosilazane with a small proportion of oxygen is particularly promising here. This material remains amorphous up to 1100° C. and crystallizes into the fluoride-ion-resistant components SiC and Si 3 N 4 . 
     Before the start of the fluoride ion cleaning, the initially liquid or spreadable precursor is applied to the non-oxidized parts of the surface to be cleaned. This may be carried out, in particular, by spreading, dripping or spraying the liquid or spreadable precursor onto the surface to be cleaned or by immersing the surface to be cleaned in the liquid precursor. A monomeric precursor, in particular, may then be initially crosslinked or partially crosslinked by means of a heat treatment. The crystallized ceramic protective layer is then produced by pyrolysis (ceramicization). 
     The pyrolysis may be carried out, in particular, in a nitrogen atmosphere. During the pyrolysis process, the organic elements burn and the material crystallizes. The fluoride ion cleaning can then be carried out, during which the corrosion and oxide residues are effectively removed from the uncoated surfaces but the coated surfaces are not exposed to the aggressive fluoride ions. 
     Once the cleaning process has been completed, the polymer-based ceramic protective layer may be removed in an alkaline bath or in an alkaline ultrasonic bath by, for example, a shot-blasting process. 
     According to the invention, polymer-based ceramic materials are therefore provided in order to be used for protecting surface portions which are not to be cleaned against fluoride ions during the cleaning of surfaces using fluoride ions. In particular, these materials can be distinguished in that the polymer-based ceramic material is a silicon-containing polymer-based ceramic material. It is possible here that the polymer-based ceramic material comprises SiC- and/or Si 3 N 4 -forming precursors. Furthermore, the precursor present in the polymer-based ceramic material may be polysilane, polycarbosilane, polysilazane, polyborosilazane or polycarbosilazane. 
     The process and the use proposed here extend the service life of the affected components considerably and therefore lead to a significant saving in terms of cost. In particular, it is now also possible to repair rotating components of a hot-gas passage without impairing the starting material. 
     By contrast with the process disclosed in U.S. Pat. No. 6,645,926, only one protective layer is required for the process described here. In particular, it is not necessary to apply a multilayered parting layer. 
    
    
     
       Further features, properties and advantages of the present invention emerge from the following description of an exemplary embodiment, with reference to the appended figures. 
         FIG. 1  shows a partial longitudinal section through a gas turbine. 
         FIG. 2  shows a combustion chamber of a gas turbine. 
         FIG. 3  shows a perspective view of a rotor blade or guide vane of a turbomachine. 
         FIG. 4  shows a schematic view of the cross section through a surface which has a crack and is partially covered by corrosion residues. 
         FIG. 5  shows the cross section from  FIG. 4 , where the non-corroded parts of the surface have been coated with a polymer-based ceramic. 
         FIG. 6  shows the cross section from  FIG. 4 , where the surface is cleaned with fluoride ions. 
         FIG. 7  shows the cross section from  FIG. 4 , where the uncoated parts of the surface show wear phenomena resulting from the fluoride ion cleaning. 
         FIG. 8  shows the cross section from  FIG. 4 , after the polymer-based ceramic protective layer has been removed and the crack repaired. 
     
    
    
     The text which follows describes  FIGS. 1-8  in detail and, with reference to  FIGS. 4-8 , provides a more detailed explanation of the process according to the invention for the gentle cleaning of partially corroded or oxidized surfaces using fluoride ions. 
       FIG. 1  shows, by way of example, a partial longitudinal section through a gas turbine  100 . 
     In the interior, the gas turbine  100  has a rotor  103  with a shaft  101  which is mounted such that it can rotate about an axis of rotation  102  and is also referred to as the turbine rotor. 
     An intake housing  104 , a compressor  105 , a, for example, toroidal combustion chamber  110 , in particular an annular combustion chamber, with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas housing  109  follow one another along the rotor  103 . 
     The annular combustion chamber  110  is in communication with a, for example, annular hot-gas passage  111 , where, by way of example, four successive turbine stages  112  form the turbine  108 . 
     Each turbine stage  112  is formed, for example, from two blade or vane rings. As seen in the direction of flow of a working medium  113 , in the hot-gas passage  111  a row of guide vanes  115  is followed by a row  125  formed from rotor blades  120 . 
     The guide vanes  130  are secured to an inner housing  138  of a stator  143 , whereas the rotor blades  120  of a row  125  are fitted to the rotor  103  for example by means of a turbine disk  133 . 
     A generator (not shown) is coupled to the rotor  103 . 
     While the gas turbine  100  is operating, the compressor  105  sucks in air  135  through the intake housing  104  and compresses it. The compressed air provided at the turbine-side end of the compressor  105  is passed to the burners  107 , where it is mixed with a fuel. The mix is then burnt in the combustion chamber  110 , forming the working medium  113 . From there, the working medium  113  flows along the hot-gas passage  111  past the guide vanes  130  and the rotor blades  120 . The working medium  113  is expanded at the rotor blades  120 , transferring its momentum, so that the rotor blades  120  drive the rotor  103  and the latter in turn drives the generator coupled to it. 
     While the gas turbine  100  is operating, the components which are exposed to the hot working medium  113  are subject to thermal stresses. The guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the direction of flow of the working medium  113 , together with the heat shield elements which line the annular combustion chamber  110 , are subject to the highest thermal stresses. 
     To be able to withstand the temperatures which prevail there, they may be cooled by means of a coolant. 
     Substrates of the components may likewise have a directional structure, i.e. they are in single-crystal form (SX structure) or have only longitudinally oriented grains (DS structure). 
     By way of example, iron-base, nickel-base or cobalt-base superalloys are used as material for the components, in particular for the turbine blade or vane  120 ,  130  and components of the combustion chamber  110 . 
     Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949; these documents form part of the disclosure with regard to the chemical composition of the alloys. 
     The blades or vanes  120 ,  130  may also have coatings which protect against corrosion (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon, scandium (Sc) and/or at least one rare earth element or hafnium). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1, 
     which are intended to form part of the present disclosure with regard to the chemical composition. 
     A thermal barrier coating, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, may also be present on the MCrAlX. 
     Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
     The guide vane  130  has a guide vane root (not shown here), which faces the inner housing  138  of the turbine  108 , and a guide vane head which is at the opposite end from the guide vane root. The guide vane head faces the rotor  103  and is fixed to a securing ring  140  of the stator  143 . 
       FIG. 2  shows a combustion chamber  110  of a gas turbine. The combustion chamber  110  is configured, for example, as what is known as an annular combustion chamber, in which a multiplicity of burners  107 , which generate flames  156 , arranged circumferentially around the axis of rotation  102  open out into a common combustion chamber space  154 . For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the axis of rotation  102 . 
     To achieve a relatively high efficiency, the combustion chamber  110  is designed for a relatively high temperature of the working medium M of approximately 1000° C. to 1600° C. To allow a relatively long service life even with these operating parameters, which are unfavorable for the materials, the combustion chamber wall  153  is provided, on its side which faces the working medium M, with an inner lining formed from heat shield elements  155 . 
     On the working medium side, each heat shield element  155  made from an alloy is equipped with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) or is made from material that is able to withstand high temperatures (solid ceramic bricks). 
     These protective layers may be similar to the turbine blades or vanes, i.e. for example MCrAlX: M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element or hafnium (Hf). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 31, EP 0 412 397 B1 or EP 1 306 454 A1, which are intended to form part of the present disclosure with regard to the chemical composition of the alloy. 
     It is also possible for a, for example, ceramic thermal barrier coating to be present on the MCrAlX, consisting for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. 
     Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
     Other coating processes are possible, e.g. atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. 
     Refurbishment means that after they have been used, protective layers may have to be removed from heat shield elements  155  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the heat shield element  155  are also repaired. This is followed by recoating of the heat shield elements  155 , after which the heat shield elements  155  can be reused. 
     Moreover, a cooling system may be provided for the heat shield elements  155  and/or their holding elements, on account of the high temperatures in the interior of the combustion chamber  110 . The heat shield elements  155  are then, for example, hollow and may also have cooling holes (not shown) opening out into the combustion chamber space  154 . 
       FIG. 3  shows a perspective view of a rotor blade  120  or guide vane  130  of a turbomachine, which extends along a longitudinal axis  121 . 
     The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor. 
     The blade or vane  120 ,  130  has, in succession along the longitudinal axis  121 , a securing region  400 , an adjoining blade or vane platform  403  and a main blade or vane part  406  and a blade or vane tip  415 . 
     As a guide vane  130 , the vane  130  may have a further platform (not shown) at its vane tip  415 . 
     A blade or vane root  183 , which is used to secure the rotor blades  120 ,  130  to a shaft or a disk (not shown), is formed in the securing region  400 . 
     The blade or vane root  183  is designed, for example, in hammerhead form. Other configurations, such as a fir-tree or dovetail root, are possible. 
     The blade or vane  120 ,  130  has a leading edge  409  and, a trailing edge  412  for a medium which flows past the main blade or vane part  406 . 
     In the case of conventional blades or vanes  120 ,  130 , by way of example solid metallic materials, in particular superalloys, are used in all regions  400 ,  403 ,  406  of the blade or vane  120 ,  130 . 
     Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949; these documents form part of the disclosure with regard to the chemical composition of the alloy. 
     The blade or vane  120 ,  130  may in this case be produced by a casting process, by means of directional solidification, by a forging process, by a milling process or combinations thereof. 
     Workpieces with a single-crystal structure or structures are used as components for machines which, in operation, are exposed to high mechanical, thermal and/or chemical stresses. 
     Single-crystal workpieces of this type are produced, for example, by directional solidification from the melt. This involves casting processes in which the liquid metallic alloy solidifies to form the single-crystal structure, i.e. the single-crystal workpiece, or solidifies directionally. 
     In this case, dendritic crystals are oriented along the direction of heat flow and form either a columnar crystalline grain structure (i.e. grains which run over the entire length of the workpiece and are referred to here, in accordance with the language customarily used, as directionally solidified) or a single-crystal structure, i.e. the entire workpiece consists of one single crystal. In these processes, a transition to globular (polycrystalline) solidification needs to be avoided, since non-directional growth inevitably forms transverse and longitudinal grain boundaries, which negate the favorable properties of the directionally solidified or single-crystal component. 
     Where the text refers in general terms to directionally solidified microstructures, this is to be understood as meaning both single crystals, which do not have any grain boundaries or at most have small-angle grain boundaries, and columnar crystal structures, which do have grain boundaries running in the longitudinal direction but do not have any transverse grain boundaries. This second form of crystalline structures is also described as directionally solidified microstructures (directionally solidified structures). 
     Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1; these documents form part of the disclosure with regard to the solidification process. 
     The blades or vanes  120 ,  130  may likewise have coatings protecting against corrosion or oxidation e.g. (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element, or hafnium (Hf)). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1, which are intended to form part of the present disclosure with regard to the chemical composition of the alloy. 
     The density is preferably 95% of the theoretical density. 
     A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX layer (as an intermediate layer or as the outermost layer). 
     The layer preferably has a composition Co-30Ni-28Cr-8Al-0.6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. In addition to these cobalt-based protective coatings, it is also preferable to use nickel-based protective layers, such as Ni-10Cr-12Al-0.6Y-3Re or Ni-12Co-21Cr-11Al-0.4Y-2Re or Ni-25Co-17Cr-10Al-0.4Y-1.5Re. 
     It is also possible for a thermal barrier coating, which is preferably the outermost layer and consists for example of ZrO 2 , Y 2 O 3 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, to be present on the MCrAlX. 
     The thermal barrier coating covers the entire MCrAlX layer. Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD). 
     Other coating processes are possible, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer. 
     Refurbishment means that after they have been used, protective layers may have to be removed from components  120 ,  130  (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the component  120 ,  130  are also repaired. This is followed by recoating of the component  120 ,  130 , after which the component  120 ,  130  can be reused. 
     The blade or vane  120 ,  130  may be hollow or solid in form. 
     If the blade or vane  120 ,  130  is to be cooled, it is hollow and may also have film-cooling holes  418  (indicated by dashed lines). 
     With reference to  FIGS. 4-8 , the text which follows provides a more detailed explanation of the process according to the invention for the gentle cleaning of partially corroded or oxidized surfaces using fluoride ions. 
     The starting point is a partially cracked and corroded surface  1  of a rotor blade in a gas turbine, as shown in  FIG. 4 .  FIG. 4  shows, in a greatly simplified and schematic form, the cross section through the wall of a rotor blade as shown in  FIG. 3 . The figure shows the base material  1  of the turbine blade, the surface  8  after a thermal barrier coating which may originally have been present has been removed, and a crack  3  with a crack surface  9  which has corrosion and oxidation residues  2 . 
     Before refurbishment, it is first necessary to remove the corrosion and oxidation residues  2 . The non-corroded component surface  8  is protected by spreading liquid polycarbosilazane (Si—C—N—(O)) onto it as a precursor for a ceramic coating. Alternatively, the polycarbosilazane may also be applied by spraying the component with the liquid polycarbosilazane or immersing it therein. Polycarbosilazane which penetrates into the crack  3  during this process can be removed from the crack  3  again before the subsequent ceramicization. The coated surface is then heated to about 150° C. to 250° C., in particular to about 200° C., whereby the polycarbosilazane is crosslinked. The crosslinking hardens the previously liquid polycarbosilazane, and it is thereby ensured that this is fixed to the surface during the subsequent process steps. The use of polycarbosilazane is advantageous since this material remains amorphous in the crosslinked state up to 1100° C. By way of example, the polycarbosilazane can also be removed from the crack in the crosslinked state. 
     After the crosslinking, the polycarbosilazane is subjected to pyrolysis, during which it crystallizes to form a ceramic protective layer of SiC and Si 3 N 4 . The pyrolysis is carried out at temperatures above 1100° C., in particular at temperatures in the range from 1100° C. to 1500° C. and preferably at temperatures of about 1300° C. The ceramic protective layer  4  applied to the component surface is shown in  FIG. 5 .  FIG. 5  shows the cross-sectional area  1  from  FIG. 4 , the component surface  8  now being coated with SiC and Si 3 N 4 . 
     The corrosion and oxide residues  2  are removed during the subsequent cleaning of the crack surface  9  using fluoride ions (see  FIG. 6 ). The non-corroded component surface is protected by the ceramic protective layer  4  during the cleaning process. 
     Whereas the crack surface  9  has been attacked by the influence of the fluoride ions during, cleaning, the ceramic protective layer of SiC and Si 3 N 4  does not react with the fluoride ions, and this provides the protective action for the component surface  8 . This is shown in  FIG. 7 .  FIG. 7  shows the cross-sectional area  1  from  FIG. 5  after the crack surface has been cleaned using fluoride ions. The crack surface which is not covered by the ceramic protective layer  4  has wear phenomena  6  resulting from the fluoride ion cleaning. However, this does not represent a further problem since the crack is subsequently eliminated by subjecting it to a soldering or welding process. By contrast, the protection afforded by the ceramic protective layer  4  means that the material properties of the component surface  8  which does not require any repairs do not deteriorate. 
     After the fluoride ion cleaning, the ceramic protective layer  4  is removed in an alkaline ultrasonic bath by a shot-blasting process. When the component has been cleaned, the crack  3  can then be repaired by soldering or welding  7 . The result is shown in  FIG. 8 .  FIG. 8  shows the cross-sectional view from  FIG. 7 , after the polymer-based ceramic protective layer has been removed. The crack  3  and the parts  6  of the cleaned by the fluoride ions crack surface which have been attacked have been repaired by welding.  FIG. 8  shows the weld seam  7 . The entire surface of the repaired component eventually does not show any wear phenomena caused by the fluoride ion cleaning. 
     Although the precursor used in the present exemplary embodiment was polycarbosilazane, other precursors are also suitable. The table below compiles some suitable precursors and the resultant ceramics. If the precursor does not contain any nitrogen, the pyrolysis is carried out in a nitrogen atmosphere if it is desirable to form Si 3 N 4 . The different precursors can also be used in combined form as a mixture. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Polymer-based 
                 Amorphous 
                   
               
               
                 ceramic 
                 phase 
                 Crystalline phase 
               
               
                   
               
             
            
               
                 Polysilane 
                 Si—C—(O) 
                 SiC, Si 3 N 4 , (pyrolysis in N 2 ), C 
               
               
                 Polycarbosilane 
                 Si—C—(O) 
                 SiC, Si 3 N 4 , (pyrolysis in N 2 ), C 
               
               
                 Polysilazane 
                 Si—C—N 
                 SiC, Si 3 N 4   
               
               
                 Polyborosilazane 
                 Si—B—C—N 
                 SiC, Si 3 N 4   
               
               
                 Polycarbosilazane 
                 Si—C—N—(O) 
                 SiC, Si 3 N 4   
               
               
                   
               
            
           
         
       
     
     The process according to the invention can also be used for cleaning corroded or oxidized surface portions which do not have any cracks. In the context of refurbishment by the application of material, surface portions of this type are refurbished by means of soldering or welding processes. In such a case, those regions to which material is to be applied in such a manner are cleaned with fluoride ions, while those regions in which material is not to be applied are protected, according to the invention, by the polymer-based ceramic protective layer.