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Effects of high pH on corrosion.
Low pH can cause so many adverse effects but what are all the possible effects of high pH?
This question arises because of limits used for pH. There is always a limit to the upper side too. For example, pH should remain within 5.5 to 7.5 in Crude distillation column. What are possible effects of pH> 8?
 
Answers
11/05/2016 A: Amol Palve, Dorf Ketal B V, amolp@dorfketal.com
Below correlation as given in one of the standard corrosion handbooks:
Minimum corrosion occurs in the range of 5.5 – 6.5 pH range
Below pH 5.0: corrosion rapidly increases with decreasing pH (increasing acid concentration). The iron sulphides deposits loosen to foul downstream components.
Above pH 7.0 : Corrosion rapidly increases with increasing pH (increasing HS-concentration)
Above pH 8.5 : The stable iron sulphide (Fes) film forms and prevent further corrosion
07/05/2016 A: Muhammad Faisal, Oman Oil Refineries & Petroleum Industry (Orpic)-Oman, engr.faisalfraz@gmail.com
Refer to trend of corrosion vs pH, normal recommended range should be 5.5 ~ 6.5. but this value is for SW Boot location? or at OVHD (ICP) or from lab sample (where analysis is to be taken at 25C)?
Also pH should be controlled at 5.5-6.5. high pH (6.8-7.3) is critical for corrosion point of view. pH(7.3-7.8) is normal (if cant be controlled between 5.5-6.5).
11/04/2016 A: Stefan van der Wal, Mettler-Toledo Process Analytics, stefan.vanderwal@mt.com
@pH > 7.6 corrosion due to heat-stable salts, ammonium bisulfide in particular, increases exponentially
11/04/2016 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Correction. pH limits are 5.5 - 6.5 (not 7.5). It is the best for Overhead system to be in slight acidic phase and protected by the filmer (to prevent acid attacks on the metal) and a neutralizer, to keep neutralizing the acids that are formed in the condensation process. In simplified explanation, Acidic corrosion starts as pitting corrosion (develop pin hole leaks). Corrosion due to high pH is more dangerous than acidic corrosion as it can cause cracks in the equipment (a phenomenon known as Caustic embrittlement). The tubes of the overhead exchangers will become like dry wood and break into large pieces which are extremely difficult to fix.
10/04/2016 A: Sam Lordo, Nalco Company, salordo@comcast.net
for crude ovhds the amount of h2s present is not significant enough to result in any real issues at the elevated ph's where as in hydroprocessing stabilizer ovhd's for example the amount of h2s can be in the 1000's of ppm and you can have some wet h2s corrosion, whereas, in a crude ovhd the h2s conc is often <500 ppm. in most cases the impact is increase tendency for emulsion in the ovhd accum drum resulting in potential water carryover into the tower (not a good thing) and a waste of neutraliser (if its injection is the reason for the higher pH