Stainless steel alloys are the material of choice for certain parts, tanks and pipes because they are safe for contact with sensitive materials. The pharmaceutical, aerospace and medical industries specify clean stainless steel for much of their equipment. Unfortunately, stainless steel is not as “stainless” or corrosion-resistant as some might hope. If it is not carefully passivated in the final step of fabrication, it may show signs of corrosion (sometimes much later and while in use).
Properly cleaned stainless steel is naturally protected from corrosion by a very thin “passive” (non-reactive with its environment) film. This passive chrome oxide layer is generated naturally upon contact with oxygen from the air. But the passive layer can be removed or scratched. Abrasives, corrosive chemicals and free iron (tiny iron particles deposited on the surface from tools) can damage the passive layer. Scratched surfaces will automatically heal by recovering the passive layer over several hours. But damage caused by caustic chemicals and free iron contamination will not regenerate.
Free iron refers to the non-alloyed iron found on the surface of stainless steel. Often, free iron remains on the metal’s surface from the original fabrication of the part. Grinding, drilling, buffing, forming, rolling and bending can contaminate a surface with free iron. These surfaces can not re-passivate spontaneously. The parts first must be carefully cleaned, then immersed in a specified nitric acid bath to dissolve the free iron and aid in the reformation of the passive layer. The passivation bath does not dissolve or harm the stainless steel. The formulas used in these baths are not the same for all steels; some require dichromate addition, higher temperature or even anodic polarization. Citric acid is commonly used as well.
After the bath and a rinsing phase, the passive layer will begin to form. It may take a couple of hours (and as much as 24 hours) for the passive layer to completely regenerate. After this 24-hour rest time, testing the passivity of the stainless steel is imperative and can be performed simply with a passivation tester meter. Proper testing of finished parts will not only satisfy quality control engineers and end users, but will also serve as validation of the entire passivation bath procedure.
A passivation meter measures the surface potential under carefully controlled conditions of constant pH. The numeric volt reading is strongly affected by surface free iron, which forms an active corrosion couple. A passivation test meter can be used with all stainless steels: 300 series, 400 series, 17-4 pH and 17-7 pH. Passivation readings can be correlated to salt-spray evaluation of corrosion resistance.
Passivation verification commonly consists of copper sulfate or ferroxyl testing or lengthy humidity and salt spray testing. Test meter kits are portable, lightweight instruments that measure the corrosion tendency of the surface of stainless steels. They can detect surface-free iron through a galvanic process that is measured as a voltage. There is no size limitation to the test piece. A typical, simple test takes only about 1 or 2 seconds. Electrical contact is made to a test piece through a moist pad. Then the sensitive test probe is pressed into the pad to complete the circuit, and the cell voltage appears on the digital meter. No hazardous chemicals are used in the process, and a passive stainless steel standard is also included in the kit.
This measurement can be repeated over the surface of the test part and at sensitive spots, such as seams and welds. Values for passive and non-passive readings are included in the instructions, and a standard 304 stainless steel coupon is provided for test comparison and instrument checkout. Samples reading “non-passive” can be passivated by running them through the passivation bath. They then should be retested. Because no training is necessary, the system can be used successfully by non-technical personnel.
Koslow, a family-run business since 1966, has developed a line of kits for the rapid field identification of materials such as steels, nickel, aluminum and titanium alloys, brasses, bronzes and space age metals. These kits have been developed through the combination of analytical chemistry, electrochemistry and metallurgy in the company’s laboratories. The kits are designed to supply timely metallurgical identification in quality control and incoming inspection departments, material storage and scrap yards, warehouses, machine and welding shops, and a host of other areas where rapid, simple and positive metal identification must be made.