Patent Publication Number: US-2015079417-A1

Title: Anti-corrosion and anti-erosion protective layer containing aluminium

Description:
The invention relates to an aluminum-containing protective layer to protect against corrosion and erosion. 
     Compressor blades of stationary GTs and aircraft turbines are usually produced on the basis of heat-treated martensitic 12%-13% Cr steels and sometimes of precipitation-hardened 16%-17% Cr steels. As protection against corrosion, the blades of the front stages are protected by means of a special aluminum pigment-containing high-temperature surface coating. 
     The disadvantage of this coating is the relatively low erosion resistance of the layer. As a consequence thereof, not only a very complicated filtering of the air but often also a regular recoating of the blades (in the course of refurbishment) are necessary. 
     To solve this problem, the protective layers used to date are inspected at regular intervals for erosion damage and corrosion protection and in the course of refurbishment removal of the coating with subsequent recoating of the blades with the same system are carried out. The length of these time intervals is greatly dependent on the ambient air and the effectiveness of filter devices. 
     This problem is to be solved. 
     The object is achieved by a layer system as claimed in claim  1 . 
     The refurbishment intervals could be lengthened by increasing the erosion resistance of the coating. 
    
    
     
       In the dependent claims, further advantageous measures which can be combined with one another in any way to achieve further advantages are listed. 
         FIGS. 1-28  show layer systems. 
     
    
    
     The figures and the description present only examples of the invention. 
       FIG. 1  shows a layer system  1  made up of a substrate  4  with a single layer  7  which contains soft aluminum particles Al having a hardness of ≦Hv50. 
     Preference is given to the harder aluminum-containing particles AL* in a layer  7 ′ ( FIG. 11 ). 
     The single layer  7 ″ can, proceeding from  FIG. 1 , preferably additionally comprise hard material particles (H), as shown in  FIG. 5 . 
     This likewise applies to a single layer  7 ′″ as per  FIG. 13  proceeding from  FIG. 11 . 
       FIG. 2  shows a layer system  1  made up of a substrate  4  with a double protective layer  7 ,  10  to protect against corrosion and erosion (corresponds to FIG.  1 +outer layer  10 ). According to this prior art, the layer  7 , which is located directly on the steel-containing substrate  4 , comprises soft aluminum particles Al and acts as sacrificial anode, whereas the upper layer  10  does not comprise any aluminum particles. In  FIG. 2 , the soft aluminum particles Al are present in the lower layer  7  and the upper layer  10  does not comprise any particles but only a matrix material. 
       FIG. 8  shows  FIG. 2  for harder aluminum-containing particles AL* in the lower layer  7 ′. 
     To increase the erosion resistance, proceeding from  FIG. 2  or  FIG. 8 , exclusively hard material particles H can be present only in the outer layer  10 ″, as shown in  FIG. 4  and  FIG. 6 . 
     A further example of the invention comprises introducing aluminum into both layers ( FIG. 3 ), with softer aluminum particles Al (“pure” aluminum) being present in the lower layer  7  and harder aluminum-containing particles AL* being present in the upper layer  10 ′, however. 
     Depending on the field of application, hard material particles H can be incorporated in the lower layers  7 ″ and  7 ″′ and in the upper layer  10 ′,  10 ″ as per  FIG. 7  proceeding from  FIG. 2  and as per  FIG. 10  (proceeding from  FIG. 8 ) or only in the upper layer  10 ″ ( FIG. 4 ,  6 ) or, as per  FIG. 9  proceeding from  FIG. 8  and as per  FIG. 12  proceeding from  FIG. 2 , only in the lower layer  7 ″′,  7 ″. 
     In addition, the upper layer can be provided with soft aluminum particles Al, harder, aluminum-containing particles AL* and/or hard material particles H or the upper layer  10   IV  can comprise only soft aluminum particles composed of aluminum or the upper layer  10   V  comprises both hard material particles H and relatively hard aluminum-containing particles AL*. 
     In  FIG. 14 , the lower layer  7  comprises soft aluminum particles Al and the upper layer  10 ″′ likewise but also hard material particles H as erosion protection. 
     In  FIG. 15 , both the lower layer  7  and the upper layer  10   IV  comprise aluminum particles, with the aluminum particles being utilized in the outer layer  10   IV  to increase erosion protection even though they give less erosion protection than hard material particles. 
     In  FIG. 16 , the lower layer  7  comprises soft aluminum particles Al and the upper layer  10   V  comprises harder aluminum-containing particles AL* and hard material particles H. 
     Proceeding from  FIG. 3 ,  FIG. 17  does not comprise any aluminum particles but instead harder aluminum particles AL* in the lower layer  7 ′. 
       FIG. 18  shows a lower layer  7 ′ comprising relatively hard aluminum-containing particles AL*, on top of which an upper layer  10 ′″ comprising soft aluminum particles Al and hard material particles H has been applied. 
     In  FIG. 19 , the lower layer  7 ′ comprises relatively hard aluminum-containing particles AL* which contribute to erosion protection when the upper layer  10   IV  has been removed since the upper layer  10   IV  contains only aluminum particles. 
     The hardest erosion protection is formed by an upper layer  10   V  in which relatively hard aluminum particles AL* and hard material particles are present, with, depending on the field of application and degree of erosion protection, relatively hard aluminum particles AL* being present in the lower layer  7 ′ ( FIG. 20 ) or hard material particles being additionally present in the lower layer ( FIG. 28 ). 
     In  FIGS. 21-24 , the lower layer  7 ″ comprises aluminum particles and hard material particles H in order to increase the erosion resistance of the lower layer  7 ″. 
     In  FIG. 21 , relatively hard aluminum-containing particles AL* are present in the upper layer  10 ′ in order to contribute both to corrosion protection and to erosion protection. 
     In  FIG. 22 , this is achieved by means of soft aluminum particles Al and hard material particles H in the upper layer  10 ′″. 
     In less erosive environments, the upper layer  10   IV  can, in comparison to  FIG. 22 , also comprise only aluminum particles ( FIG. 23 ). 
     In particular, the outer layer is protected against erosion by the upper layer  10   V  comprising aluminum-containing relatively hard particles AL* and hard material particles H ( FIG. 24 ). 
     In  FIGS. 25-28 , the lower layer  7 ″′ comprises relatively hard aluminum-containing particles AL* and hard material particles H in order to protect the substrate  4  against erosion in any event, with relatively hard aluminum-containing particles AL* being present in the upper layer  10 ′ in  FIG. 25  and erosion protection also however being ensured by hard material particles in the aluminum-containing layer  10 ″′ in  FIG. 26 . 
       FIG. 27  shows an upper layer  10   IV  which comprises soft aluminum particles Al and also contributes, in particular, to corrosion protection and only when the upper layer has been removed does the lower layer ensure both corrosion protection and erosion protection. 
     As hard material particles H, it is possible to use ceramic particles, preferably aluminum nitride and boron nitride, chromium carbide, aluminum oxide or mixtures thereof. 
     The substrate  4  ( FIGS. 1-28 ) preferably comprises a steel. 
     The increase according to the invention in erosion protection is achieved by use of relatively hard, aluminum-containing particles AL* composed of an aluminum alloy, in particular a precipitation-hardened aluminum alloy, which has a 5-10-fold greater hardness and a strength which is higher by a factor of at least 5. 
     As precipitation-hardening alloys composed of aluminum and at least one element from the group consisting of Zn, Mg, Cu, Mn, Co, preference is given to using, in particular, the following alloys: Al—Zn, Al—Zn—Mn, Al—Mn—SC, Al—Mg—Mn, Al—Mg—Zn—Cu, Al—Cu—Mn—Zn, AlZn4.5Mg1, AlZnMgCu1.5, AlZnMgCu0.5, AlMg4.5Mn. 
     However, it is also possible to employ alloys having significantly higher contents of various alloying elements, e.g. having 6%-8% of zinc (Zn). 
     Preference is likewise given to using:
         particles AL* composed of aluminum composites made up of aluminum or an aluminum alloy as matrix: Al—Si, Al—Si—Mg, Al—Mn, Al—Fe, Al—Ni, Al—Zn with an incorporated second component such as ceramic particles composed of Al 2 O 3  or SiC,   dispersion-hardening aluminum materials comprising oxides, carbides or nitrides (Al 2 O 3 , Al 4 C 3 ), comprising lithium (Li) or using intermetallic phases: AlFe 83 Co, AlFe 8 Ce 4 , AlFe 8 Zr 1 ,   aluminides: in particular NiAl, Ni 3 Al, FeAl, Fe 3 Al, TiAl, Ti 3 Al,   precipitation-hardening scandium-containing (Sc) aluminum alloys Al—Sc or Al—Sc—X alloys.       

     After baking of the aluminum pigment layer, a conductivity blasting with low intensity is generally carried out. Targeted pressure blasting using optimized parameters enables a high increase of aluminum particles to be additionally achieved. 
     The matrix material of the layers  7 ,  7 ′,  7 ″,  7 ″′,  10 ,  10 ′,  10 ″,  10 ″′,  10   IV ,  10   V  ( FIGS. 1-28 ) is preferably a chromate-phosphate binder and is preferably the same for both layers.