- Email: [email protected]
Corrosion protection of steel using organic acid sealed anodized aluminum coatings
Corrosion Protection of Steel Using Organic Acid Sealed Anodized Aluminum Coatings by Garson P. Shulman and A.J. Bauman, Alumitec Products Corp., Torrance, Calif., and Wolfgang Friimberg, AlumiPlate Inc., Coon Rapids, Minn.
M
ethods currently employed to prevent appearance of the characteristic red-brown rust and ultimate corrosive failure of iron and steel generally depend on application of protective metal coatings, either plated or dip-applied, such as zinc, cadmium, nickel, or chromium. Alternatively, nickel and chromium alloys, such as stainless steels, may be used, but often with a decline in performance involving workability, strength, and cost. In corrosive environments, early failure is to be expected unless frequent, labor-intensive paint applications are made. Corrosion-resistant aluminum alloys have been used in marine environments as the native oxide layer offers some protection, and corrosion products, being gray, are not noticeable. Anodizing creates a thicker oxide coating, which provides excellent corrosion resistance when properly sealed. It is immediately obvious that greater corrosion protection for steel than previously achieved could be attained by coating with aluminum. Indeed, over the past 40 years, attempts to deposit aluminum on steel have been actively investigated. Among the earliest was thermal decomposition of trialkylaluminum ‘,2 or aluminum salts.’ Flame spray and dip coating apply molten aluminum. Vacuum techniques, such as flash evaporation, ion vapor deposit ion, and plasma spray, apply the metal as a vapor. Early attempts at electroplating from molten salt baths were not practical. Recently, a method for electroplating using an organic bath containing trialkylaluminum/metal fluoride has been commercialized.4,” At present, the only methods in commercial use are flash evaporation, ion vapor deposition, flame spray, hot dip coating, and organic electrolyte aluminum plating. Over the same general time span, methods for increasing the corrosion reMETAL FINISHING
. JUNE 1996
sistance of aluminum have evolved from dependence on chromic acid anodizing to the use of chromate conversion coatings. Driven by the toxic and carcinogenic nature of hexavalent chromium, methods for chromate-free corrosion protection of aluminum have been developed. Similar environmental considerations have impacted use of cadmium, nickel, and chromium plating, while even galvanizing is attracting attention from regulatory authorities. A recent investigation6 of conversion coatings concluded that none of the chromate-free alternatives met the 336-hr Mil C-81706 when applied to corrosion-prone aluminum alloys. The only method for protecting them is by anodizing. The authors have shown that plating with ultrapure aluminum increases corrosion resistance and that sealing anodized corrosion prone alloys with long chain organic acids provides excellent corrosion resistance.’ We initiated an investigation to see if combining these methods and applying the combination to steel could offer long-term protection against rust.
of aluminum on all six surfaces. The panels were placed in a sulfuric acid anodizing tank. When power was applied, current rose to 3.50 A instead of the usual 25 to 35 A. The only explanation for this phenomenon is that porosity extended through the aluminum to the surface of the steel so. at the pores, no resistive aluminum oxide could form to impede flow of electricity. Because anodizing is not possible, and because corrosion would be rapid at the pinholes, no corrosion testing was conducted. FLAME SPRAY Salt water corrosion resistance of flame-sprayed steel posts coated with aluminum was found to be poor unless the coating was sealed with an organic polymer.x Based on this report, we concluded that flame-sprayed aluminum on steel would behave like IVD aluminum during anodizing. Attempts to procure and test such material were, therefore, abandoned.
HOT DIP COATING EVAPORATIVE
METHODS
Flash evaporation of aluminum onto plastic, glass, or metal surfaces is usually used to produce very thin coatings, Anodizing produces an oxide layer partly by erosion of the underlying aluminum and partly by growth outward from the original surface. This factor would severely limit the thickness of the oxide coating to less than twice the original thickness of the aluminum. Since a minimum of 0.2 mil of oxide coating is required for corrosion protection with organic sealants, no attempt was made to work with this type of material. Ion vapor deposition (ND) is capable of producing thicker, more adherent coatings. Steel panels measuring 3 X 10 X 0.62 in. were coated with 1 mil 0 Copyright
Elsevier Science Inc.
Samples of steel coated with “pure” aluminum at temperatures above the aluminum melting point were cut from a sheet supplied by Armco Steel, intended for architectural use. Panels were 3 X 10 in. The thickness of the aluminum coating was measured by microscopic observation of a cross section. It varied from 0.85 to 1.5 mil. The edges were masked with plastic, then panels were anodized until an 0.3-mil oxide layer had formed. The panels were sealed by immersion in isostearic acid 99.9%-benzotriazole O.l%, then wiped with paper towels till dry. With the mask still in place, a panel was exposed under ASTM B 117 salt spray conditions. After 840 hours, white rust had begun to spread in from the masked edges. The test was discontin93
hours. red rust (iron oxide) had begun to form. The lower third of the panel had no sign of corrosion (Fig. I). A third panel was removed after 2,000 hours for photographic purposes, with no apparent corrosion at the time this article was written, then was returned to salt spray to determine ultimate failure time.
DISCUSSION
Figure 1. Aluminum-coated steel panels after salt spray exposure: left-3 x 10 in., 840 hr; upper right-2.5 in., 2,040 hr; lower right-2.5 in., 1,340 hr. ued. The center portion of the panel had no sign of corrosion, as shown in Figure 1.
ELECTROPLATING Steel panels 2.5 X 2.5 X 0.062 in. were electroplated.4,’ The process involves conventional nickel plating followed by plating of ultrapure aluminum from an aluminum anode through a trialkylaluminum-metal fluoride-toluene electrolyte under anaerobic and anhydrous conditions. At the time, these were the largest samples that could be plated.* Panels were given a I-mil coating of aluminum, anodized to form an 0.3-mil oxide layer, then sealed with isostearic acid-benzotriazole just as the dip-coated panels were. Three panels were subjected to salt spray testing (ASTM B 117). One panel was removed at 720 hours for demonstration purposes, while still corrosion free. A second panel showed white rust (aluminum corrosion) at 840 hours, spreading from the contact point in the hole used to hang the panel during electroplating. After 1, I80 ’A rack
envelope measuring 18 X 24 X 9 in.
is now available and design and construction of a larger tank is beginning. At one time, a 12-O tank with a 5 X IO ft. rack envelope was operational in Europe.
94
Steel coated with aluminum by either dip coating or electroplating can be protected from corrosion if the aluminum coating is anodized and sealed with isostearic acid. The aluminum deposited by electroplating is extremely pure (99.99-t%) because the impurities present in the aluminum alloy anode precipitate from the electrolyte. With no posttreatment, the coating will survive approximately 1,000 hours in salt spray. Anodizing with an organic acid seal more than doubles the lifetime. Compared to nickel-, chromium-, or cadmium-plated steel, a tenfold improvement is possible. Using bright dip techniques for a shiny finish, it compares favorably with decorative chromium plating. The hardness of aluminum oxide suggests the possibility of replacing functional chromium-plated steel. Since aluminum is nontoxic, the advantages over nickel and cadmium plating are obvious, and the appearance of anodized aluminum is superior to zinc-coated steels. An 8 pm (0.3 mil) coating of electroplated aluminum is required to prevent porosity. While the plated layer could theoretically be as thin as 12 pm, to allow for formation of an 8 pm (0.3 mil) anodized coating to accept the organic seal, a thicker coating, such as the one used in this investigation, provides a safety factor. It should be stressed that early failure often is initiated at a poorly coated contact point. By racking the parts with flexible contact points, allowing some movement in relation to the rack, the issue of insufficient thickness of aluminum may be avoided. Selection of contact points on the part can also alleviate this problem. For large parts not requiring precise tolerances, and which can withstand the temperature of molten aluminum, dip coating might be preferred. Elec-
troplating is conducted at a moderate temperature, 99°C (2lO”F), which allows for a uniform thickness and should be selected for precision machined parts. We suspect that thermal decomposition coating would provide a surface more like the one resulting from dip coating. While it might be a satisfactory alternative, it would require a development program beyond the resources of the authors. Aircraft use of steel is limited by its weight. Parts such as landing gear struts or fasteners, where high strength is required, account for most of these uses. Active programs to replace cadmium and chromium plating have been the subject of recent conferences sponsored by the National Defense Center for Environmental Excellence. Aluminum coating is one potential method for replacement of these coatings. When combined with anodizing and organic acid sealing, enhanced corrosion and wear resistance can be obtained along with the phaseout of environmentally hazardous materials. Acknowledgment We thank Joe Hillock of Hillock Anodizing for anodizing the samples, Ron Christie of Tribo Coatings for help with the ion vapor deposition, and Olivia Vasquez and Ann Stone of Durkee Testing Laboratories for the salt spray work. References I. Buloff, J.J., U.S. Patent 2,847,320; 1958 2. Breining, E.R. et al., U.S. Patent 2,929,739: 1960 U.S. Patent 3. Drummond, F.E., 2.8X0,1 IS; 1959 4. Birkle, S. and K. Stoeger, U.S. Patent 4,42.5,21 I; 1984 5. Birkle. S. et al.. U.S. Patent 4,417,954; 1983 6. Alternatives to Chromium for Metal Finishing, NCMS Report 0273RE95, National Center for Manufacturing Sciences, Ann Arbor, Mich.; October 1995 7. Shulman, G.P. and A.J. Bauman, Mefnl Finisl1in,q, 937): 16; 1995 8. Kumar, A.. “The Field Performance of Coatings in Cape Cod, Massachusetts,” Proc~eedings
of’ S~mposi~rm
017 Envir-on-
m~~ntal(v
Acwptuhle
Inhihitor.~
Coatin,gs,
The Electrochemical
ut70’
Society
(in press)
Biographies Garson P. (Gus) Shulman holds a B.S. from the California Institute of Technology, METAL
FINISHING
. JUNE
1996
M.S. from Miami University, and Ph.D. from Syracuse University in Organic Chemistry. He has been a faculty member at East Texas State University and California State Polytechnic University. Industrial experience includes research at ADM, Martin-Marietta, Jet Propulsion Laboratories Alchem Laboratories, and a consulting career. He is currently President and Director of Research at Alumitec Products Corp. Research interests include chemistry of agricultural products, organic geochemistry, and materials science, particularly thermal decomposition and polymer chemistry.
Albert J. Bauman holds a B.S. in chemistry from the University of Southern California. Experience includes research at Aerojet General, City of Hope, Jet Propulsion Laboratory, Meteorology Research, and a consulting career. He is currently a consultant and research chemist at Alumitec. Research interests include lipid chemistry, organic geochemistry, military and law enforcement detection systems, and analytical instrumentation.
Bochum. After postdoctorate training at the Max Planck Institute for Coal Research, he worked in the Research and Development Department of Interatom GmbH, the nuclear energy subsidiary of Siemens AG, in charge of research for electroplated aluminum. and a similar position at Techstart, Inc. He is currently Director of Research and Development for AIumiPlate Inc.
MF
Wolfgang Fromberg holds a Doctorate in Chemistry from the Ruhr University,
Nickel and Chromium Plating, Third Edition by J.K. Dermis and T.E. Such
449 pages
$128.00
book the authors describe the industrial processes for carrying out nickel and chromium plating. New material reflects the move from purely decorative and corrosion protection of mass produced articles to surface engineering of more valuable products. Although updated throughout, new materials of particular
In thk
interest appears on environmental and health concerns, electroless nickel, electroforming, and alloy deposition.
Send Orders to: METAL IrINISHING, 660 White Plains Road, Tarrytown NY 10591 For faster service call (914) 333-2578 or fax your order to (914) 333-2570 All book orders must be prepaid. NY, NJ and MA residents add appropriate sales tax. Please include $5.00
shipping and handling for delivery of each book via UPS to addresses in the U.S., $8.00 for each book for Air Parcel Post shipment to Canada; and $20.00 for each book for Air Parcel Post shipment to aU other countries.
Aaueous --I---
-
Cleanins
-~-
-~-
-
u
For aqueous detergent cleaning, Graymills is where the action is. More cleaning actions, more power, more ways. Graymills has aqueous cleaning and degreasing equipment for almost any finishing application from in-process to final stage. Graymills designs and builds fully automatic washrinse-dry systems as well as batch and hand units.
More Actions - More Power
Graymills equipment is designed to combine multiple cleaning actions to maximize performance. Contact customer service for Graymills Corporation 8705 N. Lincoln Chicago, #Iinois 6061 S-3594
FLUUl AGITATION
SPRAY
HEAT ACTION
a catalog or product information: Phone: l-31 2-243-6325 Fax: l-31 2-477-8873 E-mail: graymillsOindustry.net
GA512 Circle 060 on reader information card METAL FINISHING
??
JUNE
1996
95
M
ethods currently employed to prevent appearance of the characteristic red-brown rust and ultimate corrosive failure of iron and steel generally depend on application of protective metal coatings, either plated or dip-applied, such as zinc, cadmium, nickel, or chromium. Alternatively, nickel and chromium alloys, such as stainless steels, may be used, but often with a decline in performance involving workability, strength, and cost. In corrosive environments, early failure is to be expected unless frequent, labor-intensive paint applications are made. Corrosion-resistant aluminum alloys have been used in marine environments as the native oxide layer offers some protection, and corrosion products, being gray, are not noticeable. Anodizing creates a thicker oxide coating, which provides excellent corrosion resistance when properly sealed. It is immediately obvious that greater corrosion protection for steel than previously achieved could be attained by coating with aluminum. Indeed, over the past 40 years, attempts to deposit aluminum on steel have been actively investigated. Among the earliest was thermal decomposition of trialkylaluminum ‘,2 or aluminum salts.’ Flame spray and dip coating apply molten aluminum. Vacuum techniques, such as flash evaporation, ion vapor deposit ion, and plasma spray, apply the metal as a vapor. Early attempts at electroplating from molten salt baths were not practical. Recently, a method for electroplating using an organic bath containing trialkylaluminum/metal fluoride has been commercialized.4,” At present, the only methods in commercial use are flash evaporation, ion vapor deposition, flame spray, hot dip coating, and organic electrolyte aluminum plating. Over the same general time span, methods for increasing the corrosion reMETAL FINISHING
. JUNE 1996
sistance of aluminum have evolved from dependence on chromic acid anodizing to the use of chromate conversion coatings. Driven by the toxic and carcinogenic nature of hexavalent chromium, methods for chromate-free corrosion protection of aluminum have been developed. Similar environmental considerations have impacted use of cadmium, nickel, and chromium plating, while even galvanizing is attracting attention from regulatory authorities. A recent investigation6 of conversion coatings concluded that none of the chromate-free alternatives met the 336-hr Mil C-81706 when applied to corrosion-prone aluminum alloys. The only method for protecting them is by anodizing. The authors have shown that plating with ultrapure aluminum increases corrosion resistance and that sealing anodized corrosion prone alloys with long chain organic acids provides excellent corrosion resistance.’ We initiated an investigation to see if combining these methods and applying the combination to steel could offer long-term protection against rust.
of aluminum on all six surfaces. The panels were placed in a sulfuric acid anodizing tank. When power was applied, current rose to 3.50 A instead of the usual 25 to 35 A. The only explanation for this phenomenon is that porosity extended through the aluminum to the surface of the steel so. at the pores, no resistive aluminum oxide could form to impede flow of electricity. Because anodizing is not possible, and because corrosion would be rapid at the pinholes, no corrosion testing was conducted. FLAME SPRAY Salt water corrosion resistance of flame-sprayed steel posts coated with aluminum was found to be poor unless the coating was sealed with an organic polymer.x Based on this report, we concluded that flame-sprayed aluminum on steel would behave like IVD aluminum during anodizing. Attempts to procure and test such material were, therefore, abandoned.
HOT DIP COATING EVAPORATIVE
METHODS
Flash evaporation of aluminum onto plastic, glass, or metal surfaces is usually used to produce very thin coatings, Anodizing produces an oxide layer partly by erosion of the underlying aluminum and partly by growth outward from the original surface. This factor would severely limit the thickness of the oxide coating to less than twice the original thickness of the aluminum. Since a minimum of 0.2 mil of oxide coating is required for corrosion protection with organic sealants, no attempt was made to work with this type of material. Ion vapor deposition (ND) is capable of producing thicker, more adherent coatings. Steel panels measuring 3 X 10 X 0.62 in. were coated with 1 mil 0 Copyright
Elsevier Science Inc.
Samples of steel coated with “pure” aluminum at temperatures above the aluminum melting point were cut from a sheet supplied by Armco Steel, intended for architectural use. Panels were 3 X 10 in. The thickness of the aluminum coating was measured by microscopic observation of a cross section. It varied from 0.85 to 1.5 mil. The edges were masked with plastic, then panels were anodized until an 0.3-mil oxide layer had formed. The panels were sealed by immersion in isostearic acid 99.9%-benzotriazole O.l%, then wiped with paper towels till dry. With the mask still in place, a panel was exposed under ASTM B 117 salt spray conditions. After 840 hours, white rust had begun to spread in from the masked edges. The test was discontin93
hours. red rust (iron oxide) had begun to form. The lower third of the panel had no sign of corrosion (Fig. I). A third panel was removed after 2,000 hours for photographic purposes, with no apparent corrosion at the time this article was written, then was returned to salt spray to determine ultimate failure time.
DISCUSSION
Figure 1. Aluminum-coated steel panels after salt spray exposure: left-3 x 10 in., 840 hr; upper right-2.5 in., 2,040 hr; lower right-2.5 in., 1,340 hr. ued. The center portion of the panel had no sign of corrosion, as shown in Figure 1.
ELECTROPLATING Steel panels 2.5 X 2.5 X 0.062 in. were electroplated.4,’ The process involves conventional nickel plating followed by plating of ultrapure aluminum from an aluminum anode through a trialkylaluminum-metal fluoride-toluene electrolyte under anaerobic and anhydrous conditions. At the time, these were the largest samples that could be plated.* Panels were given a I-mil coating of aluminum, anodized to form an 0.3-mil oxide layer, then sealed with isostearic acid-benzotriazole just as the dip-coated panels were. Three panels were subjected to salt spray testing (ASTM B 117). One panel was removed at 720 hours for demonstration purposes, while still corrosion free. A second panel showed white rust (aluminum corrosion) at 840 hours, spreading from the contact point in the hole used to hang the panel during electroplating. After 1, I80 ’A rack
envelope measuring 18 X 24 X 9 in.
is now available and design and construction of a larger tank is beginning. At one time, a 12-O tank with a 5 X IO ft. rack envelope was operational in Europe.
94
Steel coated with aluminum by either dip coating or electroplating can be protected from corrosion if the aluminum coating is anodized and sealed with isostearic acid. The aluminum deposited by electroplating is extremely pure (99.99-t%) because the impurities present in the aluminum alloy anode precipitate from the electrolyte. With no posttreatment, the coating will survive approximately 1,000 hours in salt spray. Anodizing with an organic acid seal more than doubles the lifetime. Compared to nickel-, chromium-, or cadmium-plated steel, a tenfold improvement is possible. Using bright dip techniques for a shiny finish, it compares favorably with decorative chromium plating. The hardness of aluminum oxide suggests the possibility of replacing functional chromium-plated steel. Since aluminum is nontoxic, the advantages over nickel and cadmium plating are obvious, and the appearance of anodized aluminum is superior to zinc-coated steels. An 8 pm (0.3 mil) coating of electroplated aluminum is required to prevent porosity. While the plated layer could theoretically be as thin as 12 pm, to allow for formation of an 8 pm (0.3 mil) anodized coating to accept the organic seal, a thicker coating, such as the one used in this investigation, provides a safety factor. It should be stressed that early failure often is initiated at a poorly coated contact point. By racking the parts with flexible contact points, allowing some movement in relation to the rack, the issue of insufficient thickness of aluminum may be avoided. Selection of contact points on the part can also alleviate this problem. For large parts not requiring precise tolerances, and which can withstand the temperature of molten aluminum, dip coating might be preferred. Elec-
troplating is conducted at a moderate temperature, 99°C (2lO”F), which allows for a uniform thickness and should be selected for precision machined parts. We suspect that thermal decomposition coating would provide a surface more like the one resulting from dip coating. While it might be a satisfactory alternative, it would require a development program beyond the resources of the authors. Aircraft use of steel is limited by its weight. Parts such as landing gear struts or fasteners, where high strength is required, account for most of these uses. Active programs to replace cadmium and chromium plating have been the subject of recent conferences sponsored by the National Defense Center for Environmental Excellence. Aluminum coating is one potential method for replacement of these coatings. When combined with anodizing and organic acid sealing, enhanced corrosion and wear resistance can be obtained along with the phaseout of environmentally hazardous materials. Acknowledgment We thank Joe Hillock of Hillock Anodizing for anodizing the samples, Ron Christie of Tribo Coatings for help with the ion vapor deposition, and Olivia Vasquez and Ann Stone of Durkee Testing Laboratories for the salt spray work. References I. Buloff, J.J., U.S. Patent 2,847,320; 1958 2. Breining, E.R. et al., U.S. Patent 2,929,739: 1960 U.S. Patent 3. Drummond, F.E., 2.8X0,1 IS; 1959 4. Birkle, S. and K. Stoeger, U.S. Patent 4,42.5,21 I; 1984 5. Birkle. S. et al.. U.S. Patent 4,417,954; 1983 6. Alternatives to Chromium for Metal Finishing, NCMS Report 0273RE95, National Center for Manufacturing Sciences, Ann Arbor, Mich.; October 1995 7. Shulman, G.P. and A.J. Bauman, Mefnl Finisl1in,q, 937): 16; 1995 8. Kumar, A.. “The Field Performance of Coatings in Cape Cod, Massachusetts,” Proc~eedings
of’ S~mposi~rm
017 Envir-on-
m~~ntal(v
Acwptuhle
Inhihitor.~
Coatin,gs,
The Electrochemical
ut70’
Society
(in press)
Biographies Garson P. (Gus) Shulman holds a B.S. from the California Institute of Technology, METAL
FINISHING
. JUNE
1996
M.S. from Miami University, and Ph.D. from Syracuse University in Organic Chemistry. He has been a faculty member at East Texas State University and California State Polytechnic University. Industrial experience includes research at ADM, Martin-Marietta, Jet Propulsion Laboratories Alchem Laboratories, and a consulting career. He is currently President and Director of Research at Alumitec Products Corp. Research interests include chemistry of agricultural products, organic geochemistry, and materials science, particularly thermal decomposition and polymer chemistry.
Albert J. Bauman holds a B.S. in chemistry from the University of Southern California. Experience includes research at Aerojet General, City of Hope, Jet Propulsion Laboratory, Meteorology Research, and a consulting career. He is currently a consultant and research chemist at Alumitec. Research interests include lipid chemistry, organic geochemistry, military and law enforcement detection systems, and analytical instrumentation.
Bochum. After postdoctorate training at the Max Planck Institute for Coal Research, he worked in the Research and Development Department of Interatom GmbH, the nuclear energy subsidiary of Siemens AG, in charge of research for electroplated aluminum. and a similar position at Techstart, Inc. He is currently Director of Research and Development for AIumiPlate Inc.
MF
Wolfgang Fromberg holds a Doctorate in Chemistry from the Ruhr University,
Nickel and Chromium Plating, Third Edition by J.K. Dermis and T.E. Such
449 pages
$128.00
book the authors describe the industrial processes for carrying out nickel and chromium plating. New material reflects the move from purely decorative and corrosion protection of mass produced articles to surface engineering of more valuable products. Although updated throughout, new materials of particular
In thk
interest appears on environmental and health concerns, electroless nickel, electroforming, and alloy deposition.
Send Orders to: METAL IrINISHING, 660 White Plains Road, Tarrytown NY 10591 For faster service call (914) 333-2578 or fax your order to (914) 333-2570 All book orders must be prepaid. NY, NJ and MA residents add appropriate sales tax. Please include $5.00
shipping and handling for delivery of each book via UPS to addresses in the U.S., $8.00 for each book for Air Parcel Post shipment to Canada; and $20.00 for each book for Air Parcel Post shipment to aU other countries.
Aaueous --I---
-
Cleanins
-~-
-~-
-
u
For aqueous detergent cleaning, Graymills is where the action is. More cleaning actions, more power, more ways. Graymills has aqueous cleaning and degreasing equipment for almost any finishing application from in-process to final stage. Graymills designs and builds fully automatic washrinse-dry systems as well as batch and hand units.
More Actions - More Power
Graymills equipment is designed to combine multiple cleaning actions to maximize performance. Contact customer service for Graymills Corporation 8705 N. Lincoln Chicago, #Iinois 6061 S-3594
FLUUl AGITATION
SPRAY
HEAT ACTION
a catalog or product information: Phone: l-31 2-243-6325 Fax: l-31 2-477-8873 E-mail: graymillsOindustry.net
GA512 Circle 060 on reader information card METAL FINISHING
??
JUNE
1996
95