VERISURF
Published

Determine Passivation of Stainless

Properly cleaned stainless steel is naturally protected from corrosion by a thin, passive film. But this passive layer can be removed or scratched. A passivation meter measures the surface potential under controlled conditions.

Share

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

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.

Meter Operation

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.

VERISURF
Horn USA
Techspex
SolidCAM
manufacturer of machine tools
Star swiss-type automatic lathes
Kyocera
SPC Innovations, In-machine gaging and attachments

Related Content

2023 Emerging Leaders Strengthen Their Staffs, Solve Problems

Superb critical thinking, top-notch leadership skills and a passion for building a strong team are a few of the common traits held by this year’s five Production Machining Emerging Leader award winners.

Read More

Video Tech Brief: CNC Screw Machines a Solution for Overcoming Labor Shortages

CNC screw machines can exceed job shop productivity and enable manufacturers to overcome perpetual employment gaps.

Read More
PMPA

Manufacturing Skills Training: Virginia Martinez and Laiken Carrillo

Roles of Women in Manufacturing Series: A precision machining career starts with skills. Virginia and Laiken share their journey and how they help prepare the next generation.

Read More

Succession Planning: Three 15-Minute Activities to Start Preparing for Tomorrow

Succession planning is planning for the future success of your business.

Read More

Read Next

Material Impacts on the Precision Machining Industry - August 2009

We believe that it is time to fasten our seatbelts as an industry. Inventories continue to decrease, foreign nations (Japan and Taiwan) are actively seeking to accumulate scrap, primary mills see no compelling reason to restart facilities, and the Chinese are "arresting" employees of the iron ore supplier companies that they are in "negotiations" with. Surcharges are volatile as a result of the 40-percent increase in steel scrap this month alone. Severstal announced it was ceasing all North American operations today as I prepared this report (5 August 2009). I cannot confirm that any BOF melt bar capacity is currently running in North America.

Read More
Workforce Development

Material Analysis At The Source

Growing material costs and quality control concerns have increased demand for accurate and fast methods of analyzing alloy composition. X-ray fluorescence (XRF) handheld analyzers allow in-line testing of a variety of materials.

Read More

Fielding Manufacturers’ FAQs about CMMC

Here are answers to frequently asked questions we as a provider of testing, consulting, information and compliance services receive about Cybersecurity Maturity Model Certification.

Read More
VERISURF