Can surface treatment improve the corrosion resistance of metal 3D printed parts?

一, The main part of surface treatment technology
The surface condition has a direct effect on how well metal 3D printed objects resist corrosion. Surface roughness, tiny flaws, and compositional segregation speed up the penetration of corrosive substances like chloride ions and acidic gasses. On the other hand, surface treatment methods make materials more resistant to corrosion by doing the following:
Defect removal: Get rid of surface flaws including unmelted powder particles and overlapping traces of the molten pool, and make it harder for corrosive media to stick. Chemical polishing, for instance, can get rid of a 70 μm thick sticky layer by selectively dissolving surface protrusions. This greatly lowers the likelihood of pitting corrosion.
Optimizing microstructure means changing the size of the grains and getting rid of component segregation by using heat treatment or surface modification methods. For instance, hot isostatic pressing (HIP) can make a material's density almost 100%, get rid of internal pores, and make it harder for corrosive media to get through.
To shield the metal substrate from the corrosive medium, build a thick oxide film, alloy layer, or coating on the surface. For instance, anodizing can create an Al ₂ O3 coating that is 5 to 20 μ m thick on the surface of aluminum alloys. This makes them much more resistant to salt spray corrosion.
二, The most common surface treatment approach and how it helps protect against corrosion
1. buffing with chemicals and buffing with electricity
Chemical polishing: employing powerful oxidizing acid solutions (such hydrochloric acid and nitric acid) to selectively dissolve bumps on the surface, making it smooth at the sub-micron level. After chemical polishing, the surface roughness of 3D printed titanium alloy goes from 6–12 μm to 0.2–1 μm. The critical pitting temperature (CPT) in a 3.5% NaCl solution goes up by 15 °C, which makes it much more resistant to pitting corrosion.
Electrochemical polishing: Using electrolytic processes to get nanoscale smoothness and make a passivation film at the same time. For instance, electrochemical polishing reduced the surface roughness of 316L stainless steel from 8 μm to 0.18 μm and the corrosion rate in simulated body fluids by 90%, which is what medical implants need for long-term use.
2. Changing the surface and heating it up
Heat treatment is the process of getting rid of internal tension and improving the grain structure. Annealing and quenching are two examples of this. For instance, following heat treatment, the oxidation rate of aircraft engine turbine blades at high temperatures drops by 50 °C, and their service life goes up by 20%.
Nitriding or carburizing the surface: Putting nitrogen or carbon atoms into the surface at high temperatures to make a diffusion layer that is very hard and resistant to corrosion. For instance, after nitriding, the hardness of mold steel's surface goes up to 1000–1200HV, and it can resist salt spray corrosion for more than 1000 hours.
3. Technology for coating
Physical Vapor Deposition (PVD): Putting on strong coatings like TiN and CrN to make things more resistant to wear and corrosion. For instance, following PVD coating, the oxidation rate of nickel-based alloys that were 3D printed drops by 80% at a high temperature of 650 °C.
Chemical plating/electroplating: Putting down layers of Ni-P, Ni-B, and other alloys to fill in surface flaws and make a protective film. Electroless nickel phosphorus alloy, for instance, may cut the corrosion current density of stainless steel in seawater by 95%. Its resistance to corrosion is almost as good as that of titanium alloy.
Anodizing is good for producing thick oxide layers on light metals like aluminum alloys. For instance, after rigorous anodizing, aluminum alloy parts of spacecraft can withstand salt spray corrosion for more than 5000 hours and have a melting temperature of 2320K. This meets very high environmental standards.
三, Examples of how the industry uses data and cases
In the aerospace field, GE Aviation's LEAP engine turbine blades use 3D printing and chemical polishing to make the surface smoother, going from 10 μm to 1 μm, thus making the engine 8% more aerodynamic. At the same time, HIP treatment gets rid of interior pores, which extends the high-temperature fatigue life from 5000 to 12000 cycles.
Medical implants: After electrochemical polishing, Johnson & Johnson's 3D-printed titanium alloy interbody fusion device has a surface roughness of 0.8 μm, a 90% decrease in Staphylococcus aureus adhesion, and a clinical success rate of over 95%.
Ocean Engineering: The corrosion rate of the 3D printed nickel aluminum bronze valve made by CNOOC in saltwater went from 0.5mm/year to 0.05mm/year after laser cladding and chemical nickel plating. The valve's service life was also increased by 10 times.