Stainless Steel Corrosion Resistance Improvement

Hot Salt and Stress Corrosion Cracking

Corrosion resistance has been demonstrated in hot salt and stress corrosion cracking. Development is continuing in these areas.

Room Temperature Electrochemical Testing

Minimox^™ nanocrystalline coatings were designed to reduce oxidative spalling of stainless steel and other alloys at high temperatures. Subsequent product development indicated the coating is also superior at reducing high temperature corrosion in the presence of oxygen. The treatment works by improving the density and adhesion of the thermal oxide of the base material, be it a nickel-based alloy, stainless steel, or aluminum. The chemistry of the thermal oxide also changes, often substantially increasing the surface Cr/Fe ratio.

Even though Minimox^™-coated stainless steel was not anticipated to significantly improve room temperature corrosion properties, the corrosion resistance of Minimox^™-coated and oxidized surfaces were compared via electrochemical methods. Samples of 316L were coated with the Minimox^™ nanocrystalline solution and were oxidized in air at 350^°C for 44 hours. Uncoated control samples were also oxidized in the same manner. The samples had a golden color upon removal from the furnace.

Electrochemical cells at the University of Wisconsin-Milwaukee ACE Laboratory were used to compare the accelerated electrochemical behavior of the coated and uncoated 316L, oxidized, stainless steel samples. Hanks' Balanced Salt Solution (HBSS) without Ca and Mg and without phenol red was used as the electrolyte (simulates body fluids).

When iron or steel is placed in water, corrosion begins as a reaction in which the oxidation of metal forms the anodic portion of a corrosion cell, and the consumption of oxygen forms the reduction, or cathodic, part of the reaction. When the oxidation and reduction rates are equal, there will be a voltage that characterizes the specific reaction rate or corrosion rate-that characteristic voltage is known as the corrosion potential (E_corr). The values of E_corr and other corrosion test parameters are included in the table below.

From these tests, a few general corrosion test comments can be made:

• The coated sample has a more noble (more positive) free corrosion potential (E_corr) than the uncoated sample.
• The uncoated sample has a higher I_corr and corrosion rate than the coated material.
• The larger negative hysteresis area implies if any pits grow, the pits would not be autocatalytic and that with additional exposure time, pits would not grow, but would remain the same size.