How Acid Dissolves Metals - PowerPoint Presentation

1. How Acid Dissolves MetalsDirected at Stripping ProcessesPrepared by Dave FairbournAeromet Technologies, IncSandy Utah Prepared for ITSA SymposiumWoodlands Texas9 October 2018

2. Why Strip?Repair In process flawsRemove the old coating to replace it with a new coating of better valueRecover coating materials for recycle reuseRepair a used item for reuse

3. Quotations as BackgroundDr. Wim van Ooij – deceased Prof University of Cincinnati “All corrosion is electrochemical”, All stripping I really just a corrosion eventTo be in aqueous solution, all metals must be in a Werner complexDr. Alan Bard defined the Nernst Layer or the Helmholtz layer, gave the truest most complete explanation of the reactionDr. Dennis A Jones “Principles and Prevention of Corrosion”

4. Water and Acids Strong acids are HCL, HF, HNO3, H2SO4, H3PO4When added to water, the first H+ ion comes off, forms Hydroniuim, leaving a negative ionAnions, negative ions are Cl-, F- NO3- HSO4-, H2PO4-

5. Metals and Valences Look at the Periodic Table All metals will lose one or more electrons with a minus charge and result in a positive ion with a positive valenceOther atomic species on the right hand side of the Periodic Table like Chlorine will gain an electron and become a negative anion or ionTo support a metal in water, a Werner complex must be formed which is a electrically neutral ionic bond of a metal and an onion

6. Nernst Layer or Helmholz LayerMetal loses electrons equal to valenceWater molecules are attracted to surface to eliminate the or neutralize the electrical charge Water molecule H2O are polar either slightly negative (O- side) or slightly positive (H+ side)Layer builds up to completely neutralize the charge imbalanceBecomes identical to a capacitor

7. Werner Complexes Werner was a German who did his work in the 1930’sFor more information go to WikipediaAnions are called ligandsChemists study orientation of the complexesComplexes are the metal ion you want to strip in ionic bond to the ligand or anion which comes from your bath chemical recipeIn the past most recipes have been acids dissolved in waterNewest recipes are alkaline mixtures

8. Surface Conditions Active or PassiveActive surface completely clean, no oxides easy to bond to, what the condition of the surface when we either weld or plasma sprayPassive surface is usually oxidized but you could also consider a surface covered with a chemical like oil as passive. Difficult to react, not clean Passive Surfaces are also protected from corrosion, and attack, and pitting

9. Pitting/Surface AttackAnalytical definition defines pitting as a chloride attackIf an impressed voltage is used, surface will likely be passive and protected up to 5 volts applied, then pitting will occurAll city water contains chlorideEffluent even after cleaned up, from turkey farms and chicken houses will always contain chemicals which will damage items being stripped

10. Plasma Coatings Use the METCO (Oerlikon) catalog to determine the coating recipe. Find the ligand which forms the strongest Werner Complex with the metal ion you wish to remove Coatings which are more active than the substrate can almost always be removedCoatings which are passive when compared to the substrate will usually not be easily removed, resulting in surface attack. Ex: Magnesium Cases on Helicopter engines - Use non–inorganic chemistry

11. Black Belt KPVKey Process Variables1. Agitation, the solution must be agitate. There are no engineering units for agitation. 2. Bath Temperature, hotter goes faster. Heat may also result in surface damage. 3. E value is a tabulated value found as the Open Cell Potential or Standard Elelctrode Potential 4. Recipe Which ligand is going to be used to form the Werner Complex – consumption5. Concentration of the ligand

12. Chelating Agents Ideal ligands resulting in the optimum stripping rateMost universal is EDTA, ethylene diammine tetra-acetic acidWondering if citrate ion is good chelating agent for cobaltReverse of acidic stripping, OH- ion is used for removal of chromium which has been electroplated OH is not a chelating agent, it is negative anion Vary useful for anything more active on the E-value table than aluminum

13. Stripping System Designs Must have compatible material for tanks and pumps with regards to chemical recipeHalogens (F-, Cl-, Br-, I- As-) all will attack Stainless SteelSS304 can be used for most alkaline stripping recipes Most pumps have or are available with KYNAR (PVDF) wetted agentsBest agitation is to remove the solution from the tank, heat it filter it and return it to the tank using a pumpAlternative is Flo-King pumps performing the same operation but inside the tank

14. Some ExamplesRochelle Salts BAC 5775, the ligand is the L-Tartaric acid ligandVery common in practiceWe replace with sodium citrate and sodium percarbonateSodium hydroxide is used for chromium removal, NaOH is the most common chemical used in the US (This is my opinion)HCl dilute used with graphite plate works for most MCrAlY’s Citric acid and peroxide used for WCCo or WCCoCr on Titanium substrates Local FIC on Cobalt Materials

15. Aeromet Technologies One or two inquiries per week“can you strip this”Beaker studies launched into full scale systems 50-60 Systems Done Round the WorldPW 1100 G and Rolls Royce Trent Main Engine DiskControlled Partial Strip for Chromalloy MexicaliCompressor Blades for LHT HamburgDave is Retiring if possible Hearing Chair for Deaf Persons Super accurate ammunition in partnership with John Grubbs