Recently ive been asked a few times about stainless steel rusting and contamination of stainless steel. As we all use stainless steel in the process for distilling as in boilers,water cooling fittings and incresingly in columns, bubblers . etc... I thawt i would post some info about this Its something we all should be aware about and keep an eye on as you wouldent want to be mid run when you have a failure and loose good product or at worst have a accident eg fires burns from hot liquids or electrical problems .
cheeres Caveman
Stainless Steel - Surface Contamination in Fabrication
Topics Covered
Background
Contamination by Mild Steel
Contamination by Chloride
Contamination by Carbon
Background
Stainless steels must have clean surfaces, free from contamination to achieve their optimum corrosion resistance. During fabrication care must be taken to either protect the metal surface from contamination or to restore the surface after fabrication is complete.
Three common and undesirable contaminants that are likely to be encountered during fabrication and shipping are mild steel, salt (sodium chloride) and carbon.
Any of these contaminants may imperil the chances of the oxide film protecting the underlying stainless steel.
Contamination by Mild Steel
Corrosion initiation from mild steel contamination may occur in two ways:
1. The protective oxide film on the stainless steel may be broken when the mild steel is brought into contact with the stainless steel. This contact creates a mild steel/stainless steel interface which is a corrosion cell.
2. The mild steel contamination may be at the outer surface of the oxide film on the stainless steel. Then, under moist conditions, the mild steel will corrode and cause both oxygen depletion and ionic concentration beneath the ferrous deposit. These latter effects destabilise the adjacent oxide film on the stainless steel, which then readily corrodes.
Typical causes of contamination by mild steel include:
• Using mild steel hooks, chains, and wire ropes for lifting unprotected steel materials (Suitably placed dunnage, or old fire hose, may be used to cover the lifting tackle to avoid such damage).
• Lifting unprotected stainless steel with forks and coil lifting probes that have previously lifted unprotected carbon steel.
• Dragging stainless steel over mild steel, such as the wearing strips at the back of truck trays.
• Transporting unprotected stainless steel in mild steel railway wagons.
• Contamination from falling particles of mild steel welding and flame cutting dross from higher working levels.
• Grinding dust thrown up by power tools used on mild steel.
• Contamination from tools that have previously been used on mild steel … commonly polishing tools, drills, files, and screwdrivers.
Contamination by Chloride
When contamination by common salt (sodium chloride) occurs the dissolved chloride ions enter the structure of the protective oxide film on the surface of the stainless steel. When moisture evaporates at the surface of the metal, the chloride ions concentrate on the surface and cause the protective film to break down.
Contamination by common salt can occur from:
• Sweaty hands.
• Sea spray and seawater.
• Environmental contamination, such as in butter and cheese plants where air blasting drives salt into the product.
• Salt applied to avert or melt ice (not in Australia).
• Salts added to concrete to aid setting.
• Hydrochloric acid used to clean stonework and chemically etch concrete floors.
Contamination by Carbon
Carbon contamination usually occurs when the metal is heated (e.g. during welding), under which conditions any organic materials may break down on the surface of the metal and contaminate both the solid surface and any molten metal.
Contamination by carbon can occur from:
• Pencil, paint or marking pen markings.
• Combustion of oil films, paper, organic matter, backing strip materials, bonding agents or sooty gas flames in gas welding or heat treating.
• By welding to carbon steel
Source: Atlas Steels Australia
some more links to related s/s
RUST ON STAINLESS STEEL
http://www.sperkoengineering.com/html/Rust.pdf
TUNGSTEN INCLUSION
(This is contamination of stainless steel in the tig welding process)
http://images.slidesharecdn.com/welding ... 1246079870
Pickling and Passivation
(This is a treatment done after the welding process i simply use acetone for this purpose and find it works well and dont have to worry about the other nastys in pickling agents)
http://events.nace.org/library/corrosio ... ckling.asp