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The Theory Behind Stannous Chloride as a Corrosion Inhibitor
By David Weller
A. S. Inc has a patented solution for controlling corrosion and lead leachability using stannous chloride to form a barrier between the metal pipe and the water. Our product, AS-8150, has been used successfully in St. Paul, Minnesota, and several other communities in the United States. Sampling has shown that the barrier created by stannous chloride has been successful in controlling corrosion and reducing the lead content below the EPA action level of 15 PPB. AS-8150 is inorganic, environmentally safe and NSF/ANSI 60 certified. This paper will present our theory explaining the process that occurs when AS-8150 is added to drinking water.
There are several ways lead enters into drinking water:
· oxidative corrosion.
· electrolytic cathode/anode type situations.
· pH of the system too acidic.
· organic acids or bacteria.
Other causes for corrosion include chemical interaction and the innate sensitivities of metals. Copper, lead, steel and cast iron are the materials used in most water piping systems and each has its own corrosion characteristics. Dissimilar metals in a piping system can cause preferential corrosion of pipes due to electric potentials between the metals. Oxidative agents such as chlorine, chloramines, and ozone can heighten this battery-like effect; one metal loses electrons and the other captures them. Some of these electric potentials are as high as 2 volts. The flow of water continuously removes oxidized ions, further exposing and activating the metals. In a few cases this is the only cause of corrosion in the system.
Phosphates and pH control have been the traditional methods for controlling corrosion in drinking water distribution systems to prevent lead from leaching into the water. Phosphates, especially orthophosphates, are excellent corrosion inhibitors and have proven to be effective and economical. In some tests and practical applications, orthophosphates have outperformed stannous chloride in reducing lead levels. The drawbacks to using phosphates are the environmental consequences. Phosphorous promotes the growth of algae and/or bacteria, and incurs an additional cost for wastewater treatment.
The stannous ion is the in-between oxidation state of tin and can be either oxidized or reduced depending upon the atmosphere and the ionic state of the materials nearby. Stannous ions in the water are drawn to active metal surfaces that are constantly losing electrons as they oxidize or are forced into solution by electric potentials. The positively charged tin ion is drawn to these negative surfaces and, depending on conditions, reduces the oxidized ions (rust and corrosion) and also reduces itself. This leaves behind an inactive layer of tin and a reducing atmosphere of stannous chloride very close to the surface of the pipe (i). When an oxidizing agent attacks the tin surface the stannous ions protect it, and if the tin surface is diminished it is replaced by the ongoing water treatment. There is still corrosion occurring, but the corrosion is to the tin layer now, and not to the pipe material. Our experience in seeing the interior of several systems seems to confirm this theory. After two or three months of treatment, the lead, copper and steel pipes had a gray, micron thick layer of tin on them (ii).
AS-8150 helps in maintaining the C-factor in distribution systems by reducing Iron (III) to soluble Iron (II), which aids in removing tubercles. The layer of tin sequesters the existing corrosion products, helping to prevent lead, copper and other contaminates from leaching into the water while avoiding red-water conditions at the same time.
The color of the tin layer signifies its form as alpha or beta. The alpha form of tin is gray, has a tetrahedral structure similar to diamond and is the ceramic or non-metallic form. Beta tin is white and is the metallic form. Low concentrations of stannous chloride (< .35 PPM) added to a water system seems to favor the deposition of the alpha form of tin (iii).
The corrosion-inhibiting process using AS-8150 occurs as follows:
· Tin (II) degrades to Tin (IV) and tin.
· the tin binds to the active surface and takes over the surface chemistry.
· the barrier created greatly reduces the rate at which lead and other metals leach into water.
· the Tin (IV) binds with oxygen to form stannic oxide, a soft, non-toxic particle that washes downstream.
There has been a number of studies of stannous chloride’s anti-corrosion capabilities as well as empirical results from water systems currently using AS-8150. Since October 2000, St. Paul, Minnesota, has passed all of their sampling rounds and the 90th percentile has gone from 18 PPB to 11 PPB. The food industry uses stannous chloride as an anti-oxidant and corrosion inhibitor, and stannous chloride is FDA approved in much greater concentrations than is needed for corrosion treatment (iv). All of this experience shows that stannous chloride is an effective solution for preventing damage by corrosive agents and electric potentials in drinking water distribution systems.
To summarize, AS-8150 reduces corrosion by laying down a thin layer of tin very close to the surface of the pipe and forming a barrier on cast iron, copper, steel, lead, other metals, or plastic. This layer will:
· become a layer of sacrificial material that is replaced by a continuous treatment of less than 0.35 PPM of product.
· approximate plastic and metal at the same time once it has been established.
· passivate the active metal surfaces, leave them all with the same charge, and protect them with a barrier of ceramic tin.
· convert ferric Iron (III) to a soluble form that aids in removing tubercles.
· cause oxidizing agents to have less affect as this tin layer has a relatively hard surface, is very durable and is not easily sloughed off except by chemical action or erosion.
· reduce the electrical activity of lead, copper, cast iron, and steel.
· protect without providing nutrients. Stannous chloride would be especially useful in areas that can no longer use phosphates because of the additional environmental problems.
AS-8150 is a proven corrosion inhibitor that reduces lead leachabilty and metallic corrosion without adding nutrients to the distribution system. AS-8150 decreases the likelihood of needing a phosphate removal program to minimize algae and bacterial growth in the wastewater discharge stream. AS-8150 is effective, economical and environmentally friendly.
(i) Advanced Inorganic Chemistry by Cotton & Wilkinson, 3rd Ed. Pg. 295 (Electrochemical cells)
(ii) In some cast iron systems we have seen a gray/black coating. This is likely a metallic iron sponge derived from reduced iron corrosion, and likely combined with tin. This sponge is not soft like most metallic sponge material, but a hard plastic-like material that adheres to the cast iron surface, even as a new layer.
(iii) Advanced Inorganic Chemistry by Cotton & Wilkinson, 3rd Ed. Pg. 317 (Color of tin and its form and characteristics)
(iv) Stannous chloride can be used at concentrations of 12.5 PPM in canned soda, 20 PPM in canned asparagus, and 1500 PPM in canned tuna. The typical amount used as a corrosion inhibitor in municipal drinking water systems is 0.175 PPM or less.
David Weller has a B.S. in Chemistry from the University of California at Davis, and has two patents under his name, one of which deals with using stannous chloride in other applications. In addition, Dave has worked as an industrial chemist for over 25 years and is currently employed by Custom Blenders of Oakland, California.
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