Flow accelerated corrosion – A short review

Summary

FAC is a phenomenon that results in metal loss from piping, vessels, and equipment made of carbon steel when the protective oxide layer on the metal surface is dissolved by fast-flowing water.

If your steam distribution system is constructed of carbon steel, has pH < 9 , generates two-phase flow at any location you are very much within the domain of FAC.

Following are typical conditions for FAC

1.Flow conditions: both single- and two-phase flow conditions, with water or water-steam mixture as the flowing liquid, at a temperature of > 95 degc, and flow velocity is greater than zero – Typical for fossil power plant.

2.Chemistry condition: the flowing liquid should be such that a potential difference exists between the liquid and the carbon steel pipe wall. This difference will be responsible for the dissolution of the protective oxide layer in the flowing stream facilitated by demineralized/neutral/reducing conditions.

3.Material: the pipe material must be carbon steel or low-alloy steel. FAC will be inhibited if steel has at least 0.1% chromium.

4.Flow-restricting or redirecting geometries like an orifice, sudden contraction, expansion, elbows, reducers, etc are common accelerators

Details

Flow-accelerated corrosion (FAC), also known as flow-assisted corrosion, happens when the protective oxide layer on the metal surface is dissolved by fast-flowing water. The bare metal surface oxidizes to Fe++ [iron] ion at the anode. Anode gets created on the pipe surface due to the potential difference between the bare and unaffected metal surfaces. Fe++ reduced to iron oxide at the cathode [unaffected metal surface] to recreate the oxide layer. This process goes on. The thinning of the metal surface continues. A typical anodic reaction is anodic oxidation: Iron is oxidized to Fe++, ferrous ion at the anode, Fe = Fe2+ + 2eˉ, at cathode the reduction reaction occurs when Fe+2 reduces to hydroxide and iron oxides [rust]. [Details I guess not necessary for this post]

A visual explanation of how FAC sets in a pipeline

FAC rate depends on flow velocity. It affects carbon steel in particular carrying deoxygenated ultra-pure water or wet steam. FAC does not happen in SS lines. Even a small amount of dissolved oxygen in water can prevent FAC. FAC happens when water is reducing in nature. Increasing pH can significantly reduce the FAC rate.

FAC is not erosion: Flow accelerated corrosion (FAC) is distinct from erosion and is primarily an electrochemical corrosion process aided by chemical dissolution and mass transfer. FAC has to be distinguished from erosion-corrosion because the fundamental mechanisms for the two corrosion modes are different. FAC does not involve impingement of particles, bubbles, or cavitation which cause the mechanical (often crater-like) wear on the surface. By contrast to mechanical erosion, FAC involves the dissolution of normally poorly soluble oxide by combined electrochemical, water chemistry, and mass-transfer phenomena.

Various dimensions of FAC

Mechanism: Visual: Two steps [1] High flow velocity uprooting protective metal oxide from pipe surface and [2] Iron corrosion begins at metal oxide rupture bare metal surface by red-ox reaction explained above the surface to recreate oxide. This pipe wall thinning goes on till the metal fails. FAC degradation mechanism results in thinning of large areas of piping and fittings that can lead to sudden and sometimes catastrophic failures, as well as a huge economic loss.

If your steam distribution system generates two-phase flow below 130 degc and pH < 9, you are very much within the domain of FAC

Conditions required for FAC

1.Flow conditions: both single- and two-phase flow conditions, with water or water-steam mixture as the flowing liquid, at a temperature of > 95oC, and flow velocity is greater than zero – Typical for fossil power plant.

2.Chemistry condition: the flowing liquid should be such that a potential difference exists between the liquid and the carbon steel pipe wall. This difference will be responsible for the dissolution of the protective oxide layer in the flowing stream. A high protective oxide layer [called magnetite] solubility and subsequent rapid removal of the protective oxide from the pipe surface, is facilitated by either demineralised and neutral water or slightly alkalinized water under reducing conditions.

3.Material: the pipe material must be carbon steel or low-alloy steel. The general practice recommends that for a well-designed system, FAC will be effectively inhibited if steel components are made to contain at least 0.1% chromium.

4.Flow Geometry: FAC has been observed to occur downstream of flow-restricting or redirecting geometries like an orifice, sudden contraction, expansion, elbows, reducers, etc.

Effect of temperature on FAC

Effect of temperature: General understanding

Effect of temperature on the flow accelerated corrosion rate of carbon steel in deoxygenated ammonia AVT power plant water at pH 9.04. FAC peaks at 130 degc. The maximum FAC rate is in a temperature range of 140-160°C for single-phase flows and at about 180°C for two-phase flows. For temperatures below 50°C and above 150°C, the rate is an order of magnitude less from those at the peak temperatures

Effect of pH on FAC

Effect of pH: General understanding

Increasing the pH reduces the FAC rate. As seen from the figure, increasing the pH from 8.0 to 9.0 reduces the FAC rate significantly

Effect of oxygen concentration in water: General understanding

Increasing the oxygen concentration in water reduces the FAC rate. This relationship is particularly strong for the low values of oxygen. For values greater than 150 ppb, the FAC rate is independent of the oxygen concentration.

Credit:Google