Development of a passivated stainless steel surface
A multi-step Q-passivation method for stainless steel is described. The method includes a first high voltage electropolishing step followed by as separate low voltage chromium enhancement step. The method can also include a vacuum heat treatment step that utilizes a cyclic ramp-up period so as to maintain a relatively low pressure and low temperature throughout the step. The method can also include a controlled oxidation step during which the surface is contacted with an oxygen/inert gas mixture at low oxygen partial pressure.
1. A method for treating stainless steel comprising:
immersing a surface comprising stainless steel at the surface in a first acid bath; applying a voltage of from about 1 V to about 10 V across the first bath and between the surface and a first electrode;
following, immersing the surface in a second acid bath; applying a voltage of from about 500 mV to about 1500 mV across the second bath and between the surface and a second electrode; and
following, subjecting the surface to a vacuum pressure of about 2.5×10 −6 Torr or less for a period of time of about 18 hours or more, the surface being heated at a temperature of about 400° C. or less as the surface is subjected to the vacuum pressure, wherein as the surface is subjected to the vacuum pressure, the surface is heated according to a cyclic heating process for the period of time, each cycle encompassing a temperature variation of from about 50° C. to about 100° C. on either side of an average temperature for that cycle, the average temperature for each consecutive cycle increasing toward a target temperature.
2. The method of claim 1 , further comprising following subjection of the surface to the vacuum pressure, contacting the surface with a mixture of oxygen and an inert gas, the mixture containing the oxygen in an amount of about 10% or less.
3. The method of claim 2 , further comprising heating the surface as it is contacted with the mixture.
4. The method of claim 3 , wherein the surface is heated at a temperature of from about 100° C. to about 500° C. as the surface is contacted with the mixture.
5. The method of claim 2 , wherein the inert gas is argon.
6. The method of claim 2 , wherein the mixture contains the oxygen in an amount of from about 0.01% to about 10%.
7. The method of claim 1 , wherein the first acid bath includes sulfuric acid and phosphoric acid.
8. The method of claim 7 wherein the first acid bath includes the sulfuric acid and the phosphoric acid in a ratio of from about 70:30 to about 50:50.
9. The method of claim 1 , wherein the voltage across the second bath is from about 500 mV to about 1500 mV vs. Ag/AgCl as a reference and/or a counter electrode.
10. The method of claim 1 , wherein the second acid bath includes sulfuric acid and phosphoric acid.
11. The method of claim 10 wherein the first second bath includes the sulfuric acid and the phosphoric acid in a ratio of from about 70:30 to about 50:50.
12. The method of claim 1 , wherein the voltage is established across the first bath multiple times consecutively.
13. The method of claim 1 , wherein as the surface is subjected to the vacuum pressure, the vacuum pressure is about 5×10 −7 Torr or less for the period of time.
14. The method of claim 13 , wherein the surface is heated at a temperature of about 350° C. or less for the period of time.