Surface Treatments and Coatings (Lightweight Materials)

Transcription

1 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

2 Objec&ve Potential Weight Saving Action plan: Apply lightweight metals to appropriate parts, considering material characteristics and process. Corrosion!

3 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

4 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

5 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

6 Surface morphology of CeCCs on Al alloy Low mag. 3D surface profile High mag. 6

7 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

8 OpNcal images of CeCCs deposited on Mg, Al, and Steel panels (before and aqer B117 salt spray tesnng) As-coated AZ91 AZ EGS 7 days B117 8

9 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

10 Poten&odynamic scan of CeCCs deposited Al alloy Thick coating Thin coating Thick coanngs enhance the corrosion current by an order of magnitude E (V SCE ) ic orr, thin coating ico rr, T hic k co at ing i (A/cm 2 ) Surender et.al., MS&T 10, Oct 17-21, Houston, TX

11 Impedance spectra of CeCCs deposited Al alloy 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 ) thick coating [10 (kω.cm 2 )] thin coating [100 (kω.cm 2 )] Z Re (KΩ.cm 2 ) Surender et.al., MS&T 10, Oct 17-21, Houston, TX

12 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

13 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 inch 3.5 inch Surender and O Keefe, US patent, US B2,

14 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)

15 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

16 Galvanic corrosion current between different couples w/o CeCC w CeCC Gal. Current (ma) AZ91-EGS AZ AZ31-EGS AZ EGS 16 Surender et.al., Mg. Tech., TMS 2012

17 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/ A1, 2012

18 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

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