What is passivating and pickling and why bother?
Passivation
How To Passivate
Pickling
Abused Equipment
The equipment shown above was neither maintained nor passivated. After less than one year the equipment had extensive multiple failures. Attention to detail such as stopping leaks falling onto the heat exchanger and periodic passivation would have kept this unit running for years.
Passivation.
Reason to passivate
In a modern pharmaceutical, bio-tech or food plant you may have many millions of dollars invested in stainless steel. If properly looked after, the life may be indefinite but if abused it may not make it through the first week. The size of the investment and the disruption and costs that would follow any failure demand that you look after your installed investment in the best way possible. Not only that, but increasing regulatory load demands that all your installed and operational piping and tankage be maintained in excellent condition at all times. There is nothing quite so humiliating (and expensive) as opening a tank for a government inspector and finding that the inside is rusty.
If you are new to stainless steel then I can hear you screaming “Stainless steel does not rust!” Well, sorry, but it rusts quite vigorously under certain circumstances. Rusting can be incredibly rapid if the stainless steel is exposed to chlorine. Even brand new stainless steel can rust when exposed to nothing more than rainwater. To quote Inigo Montoya "Let me explain. No, that will take too long. Let me sum up."
During manufacturing stainless steel almost always gets exposed to carbon steel, no matter how careful the manufacturer is. During installation, it is not unusual to have someone grinding carbon steel around your expensive stainless steel, in fact it is almost impossible to create a situation in which the new stainless components are not contaminated in some way. If you are installing stainless steel it is prudent to assume that it will all be contaminated. If the surface of the steel is contaminated with carbon steel, the carbon steel will rust as soon as it gets wet. Once the rusting starts, it will continue and turn into a pit and then an actual perforation. You may wipe it off but it will not stop the rusting. Once started, it continues until you take action. The action you must take is to remove all the carbon steel contamination. To do this you must clean the stainless steel very well (possibly cleaner than you cleaned anything before but at least as clean as the dishes you eat from) and then wash the steel surface in an acid that will dissolve all the carbon steel contaminating the surface. Once you have cleaned the surface and removed the carbon steel contamination (often called removing the free iron) the surface can passivate.
What is passivation
The surface of stainless steel can exist in two states. One is active and the other is passive. Whenever you machine, grind or scuff the surface or remove metal from the surface by any means, what is exposed underneath is active. In its active state, stainless steel will corrode readily. I have seen a pool of perfectly ordinary city water cause a row of pits overnight on stainless steel whose surface was active. To convert this surface to the rust resisting surface we all know and love we must expose the surface to oxygen. Ordinary atmospheric oxygen will do fine and this process is called autopassivating. During this process the atmospheric oxygen combines with the chromium in the stainless steel and forms a chrome oxide layer. This chrome oxide layer is what gives stainless steel its remarkable properties. This process will proceed on its own, in fact it is hard to stop it happening. What does prevent it are things like oil, grease and other impurities on the surface and this is why cleaning is so critical. Once cleaned it is highly desirable to remove the free iron and then get the surface to combine with oxygen as quickly as possible. My personal preference is to use nitric acid for passivation though there are circumstances where using citric is preferable. Citric is more environmentally friendly but will actually encourage the growth of certain molds and yeasts while nitric does not.
How To Passivate
The easiest way is to have a service come in and passivate your equipment for you after it has been installed. Passivation prior to installation is often a waste of money because the equipment can be re-contaminated during shipping or installation. Beware, there are some charlatans out there who will use chemicals such as EDTA or an isomer of vitamin c. These chemicals are antioxidants. Why would you use an antioxidant when what you are trying to do is to cause the surface of the stainless steel to oxidize? Not only that but because they make it sound like rocket science they charge accordingly. Before you even consider hiring someone to perform your passivation you need to acquire a couple of relatively inexpensive but very valuable documents. Read them and understand them. You will only need to get the electropolish document if you are using electropolish to passivate as well.
Passivation Standards
ASTM A 967 Passivating and Pickling
ASTM A 380 Cleaning
ASTM B912-02 Passivating using electropolish
QQP35C Mil spec for passivating now obsolete and replace by ASTM A 967
Do it yourself passivation
In essence these documents are “How To’s” for passivation. They explain exactly what needs to happen. If you decide to do it yourself then you will need these documents. If you decide to use a service then you will still need them so that you can ask good questions and you can be sure that your equipment has truly been passivated rather than given a very expensive wash in highly technical sounding chemicals that did not do the job as well as the ASTM chemicals. Avoid all proprietary mixes. No matter what it is called it is probably no different than regular citric acid but they will charge you a premium. Even when you se that Boeing engineers support the product view it with circumspection. Remember doctors used to endorse smoking.
If all you have to do is to clean the stainless steel and then give it a wash in acid, what is the big deal? Honestly, apart from using some very nasty chemicals there is no big deal. The cleaning solutions are primarily sodium hydroxide otherwise known as caustic soda. Even dilute it can cause problems if it is on your skin without being dealt with quickly. The acid used can be either dilute nitric acid or citric acid. Neither of which is bad to work with. If you decide to use nitric acid you would probably purchase concentrated acid and dilute it on site. The concentrated acid needs careful handling. Proper safety equipment is needed for handling all these chemicals at all times.
Disposal of the used chemicals should be done in accordance with all local, state and federal regulations.
In summary, passivation is highly desirable to ensure that stainless steel is rendered as corrosion resistant as possible. This will protect your considerable investment. You can passivate your equipment yourself providing you are comfortable working with the chemicals and observe all the rules for proper disposal. Watch for people charging a lot of money for “voodoo” passivation. Pixie dust is not an approved ingredient but some of the biggest names in the business are only one step removed from using it. Proprietary chemicals are expensive and unnecessary. Use ASTM suggested chemicals and save a lot of money and be sure what you are getting.
Pickling
Pickling requires that you first clean the stainless steel surface very well and then immerse the part in a mixture of nitric acid and hydrofluoric acid. Hydrofluoric acid will dissolve glass and is very dangerous. Use with extreme caution. This is a VERY aggressive solution and it will attack the surface of the stainless steel, leaving an overall white finish, if it is exposed for too long. The purpose of pickling is to remove weld color or scale from the surface of the steel. It will also passivate the steel in the process. Pickling is a great solution to some difficult problems but it should not be used unless it is necessary. If pickling can be avoided and weld color removed by other means then that is preferable but when you absolutely have to use it, then it can be very effective. See ASTM A 380 and A 967 for more information.
Author John R. Holroyd. President of Enerquip Inc. set up and ran the largest pickling, passivating and electro-polishing facility in the state of Wisconsin for six years and became know internationally as an authority on the subject.