Process Piping Materials – Module 2 – M06-036

alloys 825, 020 and 25-6MO, and INCONEL alloy G-3. For aggressive, hot phosphoric acid environments, especially those contaminated with halides, INCONEL alloys 625, 622, C-276 and 686 are selected.

3.7.13. Corrosion by Salts

When dissolved in water, salts increase their conductivity and thereby are able to carry higher corrosion currents. Therefore, galvanic effects are more pronounced in salt solutions than in pure water. Salts may be categorized and their corrosive characteristics defined in table below:

The most commonly used nickel alloys in process environments containing sulfate salts are INCOLOY alloys 25-6MO, 825 and 020 and INCONEL alloys G-3, 625, 622, C-276 and 686. For processes using chloride salts, the most commonly used materials are MONEL alloy 400 (for reducing conditions), INCOLOY alloys 25-6MO, 825 and 020, and INCONEL alloys G-3, 625, 622, C-276 and 686.

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x Nitric Acid: Nitric acid is a strongly oxidizing and, because of this, alloys with

Process Piping Materials – Module 2 – M06-036

3.7.14. Chloride Environment

Dry chlorine is not particularly corrosive at ambient temperatures. Chlorine gas reacts with the water to form equal parts of hypochlorous and hydrochloric acid. Hypochlorous acid is an oxidizing acid and bleaching agent which is reduced to hydrochloric acid in the bleaching reaction. This combination of an oxidizing and a non-oxidizing acid is responsible for the corrosive effect of moist chlorine on metals.

Chloride corrosion can be severe in an acidic solution with a pH ≤ 4.5.

x Stainless Steels

The 300 series SS are susceptible to pitting and under deposit corrosion in chloride containing solutions. However, the most significant threat is from chloride induced-stress corrosion cracking (Cl-SCC) when the operating temperature is above the threshold temperature for SCC. To reduce the potential for pitting and ClSCC, the following precautions are recommended for 300 series SS.

304L/316L series SS materials (e.g., plates, piping, forgings, fittings, etc.) are typically specified to be supplied in the solution annealed condition in order to minimize residual stresses that can contribute to CI-SCC.

Bulk 300 series SS piping components are generally externally coated and provided with end caps/plugs as an added precaution against pitting during transport and storage. During transport, piping will be shipped and stored inside in a closed dry environment wherever possible. Deck mounted equipment, pipe and components should be avoided for ocean shipment.

Hydrostatic testing water quality should be controlled to reduce chloride concentration to an acceptable level. Chloride levels should be 50 mg/l for austenitic stainless steel and 100 mg/l for carbon steel.

Introduction of chlorides into the process system is mitigated by the use of demineralized water for absorber tower washing and solvent make-up.

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Process Piping Materials – Module 2 – M06-036

x Nickel Alloys

Nickel alloys are outstanding in their resistance to chlorine and hydrogen chloride at elevated temperatures. Although chlorine is a strong oxidizer and will combine directly with metals, at lower temperatures this reaction is so sluggish that dry chlorine can be shipped in steel. As the temperature is increased, this reaction rate increases slowly until a critical point (which varies with the metal under consideration) is reached.

MONEL alloy 400 is a standard material for trim on chlorine cylinder and tank car valves, for orifice plates in chlorine pipe lines, and for various parts of chlorine dispensing equipment. Wet chlorine at temperatures below the dew point, or aqueous solutions containing considerable amounts of free chlorine, are very corrosive to all of these alloys, except INCONEL alloy C-276 which is used for valve stems in MONEL alloy 400-seated valves to combat the effects of the ingress of moisture.

3.7.15.Water Corrosion

Corrosion rates of carbon steel piping are assessed using the Langelier Saturation Index (LSI) and other published corrosion production tools. In low corrosive waters where the flow is stagnant, a 1.5 mm corrosion allowance is adequate but 3 mm is called out for conservatism in CS piping and equipment.

For higher corrosive waters, corrosion assessment will consider temperature, oxygen concentration, halide concentration, and other pertinent factors.

Underground piping may be non-metallic. Corrosion of underground water piping will be mitigated by use of HDPE.

Demineralized water is corrosive to CS.

Above ground piping will be fabricated from 304L SS (or 316L SS), no corrosion allowance will be added.

3.7.16. Microbiological Induced Corrosion (MIC)

MIC is a form of corrosion caused by living bacterial. It is often associated with the presence of tubercles or slimy biofilms. MIC Corrosion is usually observed as localized pitting, sometimes under deposits or as tubercles that shield the organisms.

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