Transcript of Corrosion Control Iron’s nasty side…rust All metals corrode There are various types of corrosion...
- Slide 1
- Slide 2
- Corrosion Control Irons nasty siderust 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)
- Slide 3
- Galvanic Cell From Wikipedia
- Slide 4
- 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
2 + 4 e - + 2 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 2+ + 2 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 3+ + 2 O 2 This slide is
taken directly from Wikipedia
- Slide 5
- Corrosion of CI pipe (continued) In addition, the following
multistep acid-base reactions affect the course of rust formation:
Fe 2+ + 2 H 2 O Fe(OH) 2 + 2 H + Fe 3+ + 3 H 2 O Fe(OH) 3 + 3 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
- Slide 6
- 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.
- Slide 7
- 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 1940s in most cases
- Slide 8
- 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.
- Slide 9
- Corrosion Control Irons nasty siderust 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)