Many metals protect themselves from corrosion by forming a passivation layer on their outer surface. This protective layer is composed of metal oxide or metal nitride and is very thin. For stainless steels, the layer may be only 1-3 angstroms thick and for metals such as aluminum and titanium, the layer may be up to 50 angstroms thick. (A human hair is 500,000 angstroms thick for comparison). Passivation can occur naturally with exposure to oxygen or can be assisted by various passivation techniques such as electrolytic anodization, plasma treatment or chemical treatment. Due to the limited thickness of the passivation layer, the substrate being passivated must be very clean to begin with to prevent discontinuities in the passivation layer which would then compromise the protective nature of the passivation layer.
Machining, cutting, welding, polishing or otherwise “working” a metal substrate generally introduces contaminants which prevent the formation of a uniform passivation layer. Even particulate in the shop air can contaminate the metal surface and prevent uniform passivation. Magnus Engineered Equipment can supply the proper equipment to clean and passivate the metal substrates.
Laser or plasma cutting stainless steel or other metals can produce a dross which needs to be removed for part functionality and proper passivation. The same is true for stainless steel or other metal discoloration that occurs from welding. Often, aggressive etchants such as hydrofluoric acid and nitric acid are used to remove dross and less aggressive chemistry can be used to remove discoloration. The use of ultrasonics is useful for removing dross and discoloration and the 20 kHz ultrasonics offered by Magnus Engineered Equipment is the most powerful ultrasonics in the cleaning industry.
Contact Magnus Engineered Equipment to see how we can assist with your metal passivation, dross and discoloration removal needs.
Sand, Core and Shell Investment Casting Removal Using Caustic Leaching
Magnus Engineered Equipment (MEE) has supplied equipment to the investment casting industry for more than 50 years. Investment casting, or die casting, is a technique for forming metallic components having complex geometries, especially hollow components. Hollow cored super alloy airfoils and gas turbine engine components are examples of complex geometries produced with the investment casting process. MEE equipment is used with concentrated caustic solutions of NaOH (sodium hydroxide) or KOH (potassium hydroxide) to remove shell residues from ferrous and non-ferrous metal castings (including aluminum). Operating at just below the boiling point of the solution, the process breaks down the fused silica shell used in the investment casting process without damaging the metallic substrate. The process is faster, safer and more efficient than mechanical methods of shell removal and all surfaces within the substrate are cleaned.
MEE supplies equipment which uses heat, agitation and turbulation to quickly and effectively remove the shell residues from substrates including the complex geometries found in many castings. The effect of agitation and turbulation increases the dissolution rate by increasing chemical mass transfer at the shell interface. Agitation and turbulation act to quickly remove dissolved silica and spent chemistry from the substrate surface while bringing in fresh chemistry to continue the dissolution reaction at the fastest rate. MEE supplied equipment also continuously filters the bath thus removing dissolved silica and enhancing the dissolution rate of the remaining ceramic shell by returning clean chemistry to the part surface.
The temperature at which the caustic solution is operated is dictated by the concentration of the NaOH or KOH used. As a rule of thumb, it is advisable to operate at a temperature which is 20 oF below the boiling point of the solution being used so as to prevent cavitation in the pumps used to circulate the solution. (The lower pressure on the suction side of the pump lowers the boiling point of the solution). Most users of MEE equipment operate with a NaOH or KOH solution concentration of 35 to 50 wt%.
MEE supplies off-the-shelf or custom designed equipment for investment casting removal applications. Material handling, by-product removal, automation and post-processing needs can be addressed by MEE to make the overall process efficient and worry-free. Contact MEE today to discuss your investment casting application.