Corrosion Control Iron’s nasty side…rust All metals corrode There are various types of corrosion – Physical corrosion, or “erosion corrosion” – Stray current erosion (careful disconnecting the meter!) – Uniform corrosion (a problem for Knights?) – Pitting (typical with copper pipe on Hot Water lines) – Galvanic corrosion (dissimilar metals) – Bacterial corrosion (what happens at the pipe wall stays at the pipe wall)

Galvanic Cell From Wikipedia

Corrosion of Iron (cast iron pipe) The rusting of iron is an electrochemical process that begins with the transfer of electrons from iron to oxygen. The key reaction is the reduction of oxygen O e H 2 O → 4 OH - Because it forms hydroxide ions, this process is strongly affected by the presence of acid. Corrosion of most metals by oxygen is accelerated at low pH. Providing the electrons for the above reaction is the oxidation of iron that may be described as follows: Fe → Fe e − The following redox reaction also occurs in the presence of water and is crucial to the formation of rust: 4 Fe 2+ + O 2 → 4 Fe O 2− This slide is taken directly from Wikipedia

Corrosion of CI pipe (continued) In addition, the following multistep acid-base reactions affect the course of rust formation: Fe H 2 O ⇌ Fe(OH) H + Fe H 2 O ⇌ Fe(OH) H + as do the following dehydration equilibriums: Fe(OH) 2 ⇌ FeO + H 2 O Fe(OH) 3 ⇌ FeO(OH) + H 2 O 2 FeO(OH) ⇌ Fe 2 O 3 + H 2 O This slide is taken directly from Wikipedia

Summarizing… The corrosion products are dictated by the availability of water and oxygen. With limited dissolved oxygen, iron(II)- containing materials are favored, including FeO and black lodestone (Fe 3 O4). High oxygen concentrations favor ferric materials. The nature of rust changes with time, reflecting the slow rates of the reactions of solids. Furthermore, these complex processes are affected by the presence of other ions, such as Ca 2+, both of which serve as an electrolyte, and thus accelerate rust formation, or combine with the hydroxides and oxides of iron to precipitate a variety of Ca-Fe-O-OH species.

Lead and Copper Lead and copper carbonate precipitates form on the walls of lead and copper pipe, greatly reducing the corrosion of these metals The above “passivation” process explains why houses targeted for lead and copper monitoring are either relatively new homes (built in the last 10 years) or very old homes (built before the 1940’s in most cases

pH control for DBP formation Because of the conflict between high pH water favoring DBP formation, many utilities must chose an operating range for pH that addresses both corrosion control and DBP control. The use of corrosion inhibitors (like polyphosphates) allow a utility to keep the pH of water in the distribution system rather low (~7.5) without risking the negative effects of corrosion. Phosphates tend to promote passivation film on pipe walls, and can “sequester” (or envelope) iron ions and retard the corrosion process.

Corrosion Control Iron’s nasty side…rust All metals corrode There are various types of corrosion – Physical corrosion, or “erosion corrosion” – Stray current erosion (careful disconnecting the meter!) – Uniform corrosion (a problem for Knights?) – Pitting (typical with copper pipe on Hot Water lines) – Galvanic corrosion (dissimilar metals) – Bacterial corrosion (what happens at the pipe wall stays at the pipe wall)