Surface Treatments and Coatings (Lightweight Materials) Surender Maddela Ph.D., Lightweight Materials Processing, GM Research and Development Center, Warren, MI- USA Materials Research Center, Missouri University of Science and Technology (Missouri S&T), Rolla, MO- USA August 19, 2015
Objec&ve Potential Weight Saving Action plan: Apply lightweight metals to appropriate parts, considering material characteristics and process. Corrosion!
Magnesium, and Aluminum Alloys Ø Low density, High strength-to-weight ratio Weight reduction Ease of manufacturing Fuel efficiency Steel baseline design 20 Parts & 57.1 kg Coating & Corrosion resistance 10% weight reduction; increase 8% fuel efficiency 44.3 Steel kg baseline mass design reduction 110 Parts & 99.6 (44.5%) kg 63 part reduction (57.3%) GMX322 (2008 Cadillac CTS) 14.2 kg mass reduction (24.9%) 2 part reduction (10%) Magnesium design 18 Parts & 42.9 kg Mg-intensive design 47 Parts & 55.3 kg Ref. USAMP MFERD April 16, 201
PotenNal issues and solunon Ø Corrosion Active metal Galvanic corrosion Mixed metal body-in-white assembly Al and Mg dissolve acidic phosphate baths (Poor coatings) Mg panel/fastener road exposure 338 days * As-coated 168 hrs Ø Isolation by applying adhesive and sealer Ø Conversion coatings (Cerium conversion coatings) CeCCs AZ91D panels * USAMP MFERD April 16, 2010
Corrosion protec&on of Al alloys Ø High strength Al alloys susceptible to localized corrosion in chloride containing environments Ø Chromate-based coatings utilized as corrosion resistant coatings Chromate-based conversion coatings (CCCs) Chromates are carcinogenic and toxic AA2024-T3 1 hr salt spray Ø Potential replacement for CCCs is cerium-based conversion coatings (CeCCs) Shown capable of meeting military salt spray testing specifications CeCCs can be deposited by o Spontaneous and non-spontaneous CCCs 336 hrs salt spray CeCCs 336 hrs salt spray
Surface morphology of CeCCs on Al alloy Low mag. 3D surface profile High mag. 6
SEM/FIB cross- sec&on of CeCCs deposited on Al alloy SEM images Cracks Acid CeCC Alkaline FIB cross-section Substrate Pt dep. Thickness Substrate Acid then alkaline Alkaline then acid
OpNcal images of CeCCs deposited on Mg, Al, and Steel panels (before and aqer B117 salt spray tesnng) As-coated AZ91 AZ31 6016 EGS 7 days B117 8
SEM images of cerium coated aluminum alloy panels 2.1 µm 500 nm Acid Alkaline Thick coating Thin coating Acid then alkaline Alkaline then acid Surender et.al., MS&T 10, Oct 17-21, Houston, TX
Poten&odynamic scan of CeCCs deposited Al alloy 0.2 0.0-0.2 Thick coating Thin coating Thick coanngs enhance the corrosion current by an order of magnitude E (V SCE ) -0.4-0.6-0.8 ic orr, thin coating ico rr, T hic k co at ing -1.0 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 i (A/cm 2 ) Surender et.al., MS&T 10, Oct 17-21, Houston, TX
Impedance spectra of CeCCs deposited Al alloy -120-100 Thick coating ( 2.1 µm) Thin coating ( 500 µm) Thin coatings enhance the corrosion resistance of 2024-T3 alloy -80 Z In (KΩ.cm 2 ) -60-40 thick coating [10 (kω.cm 2 )] thin coating [100 (kω.cm 2 )] -20 0 0 20 40 60 80 100 120 Z Re (KΩ.cm 2 ) Surender et.al., MS&T 10, Oct 17-21, Houston, TX
1500 hrs salt spray tested UV- curable coa&ngs deposited onto CeCCs deposited Al alloy panels Thick coating Thin coating Thin coanngs, no evidence of blisters and flecking Surender et.al., MS&T 10, Oct 17-21, Houston, TX
SchemaNc diagrams of bimetallic and trimetallic couples (BIW assembly) Al rivet Steel/Al alloy Mg alloy 1 inch 2 inch 2 inch Bimetallic couple Al rivets 1.5 inch 0.5 inch 1.5 inch 0.5 Trimetallic couple Electro-galvanized steel Mg alloy (AZ91/AZ31) Al alloy (6016) 1.25 inch 1.75 inch 0.5 13 2.5 inch 3.5 inch Surender and O Keefe, US patent, US008187439B2, 2012 13
Galvanic series in sea water Active Noble Graphite Platinum Titanium SS 316,317 (passive) Cu-Ni alloys SS 304 (passive) Silver Nickel Lead Bronzes Cu Sn brasses SS 316, 317 (active) mild steel cast iron Al alloys Zn Mg alloys Cathode (Al) Anode (ss)/ Cathode (HDG steel) Anode (Al) Occurs when dissimilar metals are in contact + electrolyte solution (e.g. salt water) Electrochemical process (similar to a battery) lead to metal loss in anode Mitigation Method Avoidance of dissimilar metals Isolation of dissimilar metals (Adhesives) Sealing out electrolyte solution (Adhesives, E-Coat) EMF series not practical as it provides reversible potentials under conditions of ideality Galvanic series constructed based on corrosion potential of materials in a particular environment (generally sea water)
OpNcal images of trimetallic (BIW assembly) couples Before salt spray Blisters CoaNng and corrosion protecnon of simulated BIW assemblies demonstrated Bare panel W CeCCs E-coat w/o CeCCs E-coat w CeCCs After salt spray 4 days 7 days 14 days 14 days Surender et.al., Magnesium, TMS 2013 15
Galvanic corrosion current between different couples 1.6 1.4 1.2 w/o CeCC w CeCC Gal. Current (ma) 1.0 0.8 0.6 0.4 0.2 0.0 AZ91-EGS AZ91-6016 AZ31-EGS AZ31-6016 6016-EGS 16 Surender et.al., Mg. Tech., TMS 2012
New E- coat process (Body- in- white (BIW) assembly, automo&ve applica&ons) New E-coating electrolyte with adhesion promoter Existing Phosphating/E-coating process New E-coating process Surender, et. al., US patent, US 2012/0205011A1, 2012
Standard vs UV- curable coa&ngs Standard versus Ø Air cured (hours) Ø VOCs Primer: 340 g/l or less Top coat: 420 g/l or less Ø Cr 6+ compounds Polyurethane Top Coat UV-curable coatings Ø UV light cured (seconds) Ø No VOCs, TRI chemicals, or HAPs Ø No Cr 6+ compounds StronNum Chromate Epoxy Primer Chromate Conversion CoaNngs Multifunctional UV-Curable Coatings Cerium Conversion CoaNngs Metallic Substrate Metallic Substrate Conventional three layer coating system UV-curable two layer coating system