Abstract:
A process of preventing crack formation in chemical conversion coating comprising providing a substrate; introducing a chemical conversion coating material; adding an additive containing a binder to the conversion coating material; and coating the substrate with the chemical conversion coating and the additive.

Description:
BACKGROUND 
       [0001]    The present disclosure is directed to the use of binders that prevent crack formation in chemical conversion coatings when those coatings cure and dry to assist with the protective film formation on metal surfaces. 
         [0002]    The oxidation and degradation of metals used in aerospace, commercial, and private industries are a serious and costly problem. To prevent the oxidation and degradation of the metals used in these applications an inorganic protective coating can be applied to the metal surface. This inorganic protective coating, also referred to as a conversion coating, may be the only coating applied to the metal, or the coating can be an intermediate coating to which subsequent coatings are applied. 
         [0003]    Chromate based coatings are currently used as inorganic conversion coatings because they provide corrosion resistant properties and adhesion to subsequently applied coating layers. However, due to environmental and health and safety concerns over use of chromate based compounds, there is a need for an environmentally safer and less hazardous replacement that provides equivalent corrosion inhibition and protection to the underlying metal substrate surface, as well as adhesion to subsequently applied coatings. 
         [0004]    Chemical Conversion Coating processes have been developed that use trivalent chromium instead of hexavalent chromium species due to changes in environmental regulations and in order to prevent exposure of workers in the metal finishing industry to hexavalent chromium species. 
         [0005]    Conversion coatings are typically applied at very low film thicknesses. Slight defects in these very thin films will expose the underlying metal to the corrosive environment and compromise the conversion coating&#39;s corrosion inhibition and protection function, leading to substrate corrosion damage, such as pitting in the case of aluminum alloy substrates. 
       SUMMARY 
       [0006]    In accordance with the present disclosure, there is provided a process of preventing crack and pin-hole formation in chemical conversion coating films comprising: providing a substrate; introducing a chemical conversion coating material; adding a binder to the conversion coating material; and coating the substrate with the chemical conversion coating and the additive. 
         [0007]    In another and alternative embodiment, the binder is selected from the group consisting of polyvinyl-alcohol, polyvinylpyrrolidone, polyvinyl-acetate, styrene-butadiene and hydro-propyl-cellulose. 
         [0008]    In another and alternative embodiment, the process further comprises strengthening the chemical conversion coating during formation by reducing differential stresses caused during drying and curing of the chemical conversion coating with the binder. 
         [0009]    In another and alternative embodiment, the process further comprises binding transition metal ions in the chemical conversion coating. 
         [0010]    In another and alternative embodiment, the process further comprises preventing cracks in the chemical conversion coating. 
         [0011]    In another and alternative embodiment, the chemical conversion coating comprises trivalent chromium. 
         [0012]    In another and alternative embodiment, the process further comprises a mixture of the binders. 
         [0013]    In accordance with the present disclosure, there is provided a component comprising a substrate having a surface; and a chemical conversion coating bonded to the surface, wherein the chemical conversion coating comprises a binder configured to prevent crack formation responsive to curing. 
         [0014]    In another and alternative embodiment, the chemical conversion coating is a composite of trivalent chromium and binder. 
         [0015]    In another and alternative embodiment, the binder is selected from the group consisting of polyvinyl-alcohol, polyvinylpyrrolidone, polyvinyl-acetate, styrene-butadiene and hydro-propyl-cellulose. 
         [0016]    In another and alternative embodiment, the binder is configured to bind transition metal ions in the chemical conversion coating. 
         [0017]    In another and alternative embodiment, the component comprises a metal material. 
         [0018]    In another and alternative embodiment, a strength of the chemical conversion coating having the binder is greater than a tensile stress of the chemical conversion coating generated during curing. 
         [0019]    In another and alternative embodiment, the component further comprises a mixture of the binders. 
         [0020]    In another and alternative embodiment, the chemical conversion coating comprises trivalent chromium. 
         [0021]    Other details of the use of binders that prevent crack formation in chemical conversion coatings are set forth in the following detailed description and the accompanying drawing wherein like reference numerals depict like elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a schematic representation of a conversion coating applied to a substrate; 
           [0023]      FIG. 2  is a schematic representation of the conversion coating of  FIG. 1  after a period of curing; 
           [0024]      FIG. 3  is a schematic representation of a conversion coating having an additive applied to a substrate; 
           [0025]      FIG. 4  is a schematic representation of the conversion coating with additive of  FIG. 3  after a period of curing. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Referring now to  FIG. 1 , there is illustrated a component  10 . The component  10  includes a metal substrate  12 , for example aluminum alloy, with a surface  14 . 
         [0027]    Applied to the surface  14  of substrate  12  is a chemical conversion coating film  16 . The chemical conversion coating  16  can be very thin with for instance a thickness of about 200 nm. The chemical conversion coating  16  can be viewed as a thin membrane of the chemical conversion coating material that forms during the immersion process or during touch-up and repair activities. 
         [0028]    The chemical conversion coating  16  can be applied as a solution or dispersion. The thin film after application to surface  14  can be dried and cured under controlled or uncontrolled humidity conditions. 
         [0029]    Referring to  FIG. 2 , the thin film of the chemical conversion coating  16  applied on the surface  14  of substrate  12  has complex mechanical properties that change over time during drying and curing. Evaporation of water from the as applied film  16  during drying, as well as coating curing leaves voids and makes the film shrink both in the perpendicular and lateral to the substrate surface  14  directions  18  and  20 . The above lead to the development of pin-holes and cracks  22  in the dried and cured coating film  16 , that leave the substrate surface  14  directly exposed to the corrosive environment and make it susceptible to corrosion damage. 
         [0030]    Referring now to  FIGS. 3 and 4 , a component  110  includes a substrate  112  having a surface  114 . A chemical conversion coating film  116  comprising an additive  118  is applied to the surface  114 . The additive  118  includes a binder material system  120 . The additive  118  can be added to any variety of solutions or dispersions that are used as chemical conversion thin films  116  used for corrosion protection of the surface  114 . 
         [0031]    The binder materials  120  reduce the stresses in the film  116  when the solution of the chemical conversion coatings  116  dry and cure. The binders  120  strengthen the gel-like network of the corrosion protective coatings  116  during formation and reduce any differential stresses that originate from drying of thin films with non-uniform structures. The binders  120  can polymerize when exposed to chemical agents or radiation and can create a strong bond within the film  116  and between the film  116  and the underlying metal alloy substrate  112 . 
         [0032]    After being deposited, as the film  116  dries, it shrinks in all directions  124 , both laterally and perpendicularly to the metal surface  114 . However, due to the binder  120 , the film  116  does not form cracks. The film coating  116  remains completely covered over the surface  114 . 
         [0033]    In an exemplary embodiment, the additive  118  can comprise a binder  120  comprising poly-vinyl alcohol (PVA) having a molecular weight of about 72,000 g/mol can be dissolved in water at a concentration of about 0.1-3.5 g/100 cubic centimeters (cm3). The solution can be added to the media that is used to coat the surface of the metal substrate with the chemical conversion coating  116  corrosion protection so that the PVA concentration will be about 0.02-1.4 g/100 cubic centimeters (cm3). 
         [0034]    In another exemplary embodiment, the additive  118  can comprise a binder  120  comprising hydro-propyl-cellulose (HPC) having a molecular weight of 105 g/mol. HPC can be added to a solution with PVA binder in order to further control the drying stresses in the thin film chemical conversion coating  116  used for protection to the underlying metal surface  114 . The HPC concentration in water is about 0.35 g/100 cm3, which can be diluted further to about 0.12 g/100 cm3 by mixing with the solution that contains the chemical conversion coating  116 . 
         [0035]    In another exemplary embodiment, another type of binder  120  can be polyvinylpyrrolidone (PVP), also at about 0.5 g PVP per 100 cm3. The PVP can bind transition metal ions, such as, trivalent chromium, or zirconium ions. Other exemplary binders  120  can include styrene-butadiene binder and polyvinyl-acetate. 
         [0036]    A benefit of the additive  118  with the chemical conversion coating  116  is that trivalent conversion coatings with a binder system  120  will have less corrosion damage in a certain time frame. The trivalent conversion coatings with the additive having a binder system  120  will thereby provide a better protection of the metal substrate  112  against corrosion. 
         [0037]    The binders  120  that have been discussed in the detailed description are present in a low concentration but play a critical role in making the thin films of the chemical conversion coating  116  more elastic and the binders  120  thereby prevent the thin films from cracking during drying and curing. 
         [0038]    There has been provided the use of binders that prevent crack formation in chemical conversion coatings. While the use of binders that prevent crack formation in chemical conversion coatings has been described in the context of specific embodiments thereof, other unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations which fall within the broad scope of the appended claims.