International Journal of Scientific Research in ...ijsre.in/Vol-1/Issue-2/4.pdfFossil and Combined...

A Review on Flow-Accelerated Corrosion (FAC) in

Fossil and Combined Cycle- HRSG Plants

Bharatsinh Parmar

Lecturer, Department of Mechanical Engineering, Bhailalbhai and Bhikhabhai Institute of

Technology, Vallabh Vidyanagar, Gujarat, 388120, India.

Corresponding Author Address:

Mr. Bharatsinh R. Parmar

Lecturer,

Department of Mechanical Engineering,

Bhailalbhai and Bhikhabhai Institute of Technology,

Vallabh Vidyanagar, 388120, Gujarat, India.

ABSTRACT

Flow-accelerated corrosion (FAC) has been researched for over 40 years at many locations

around the world, and scientifically all the major influences are well recognized. However, the

application of this science and understanding to fossil and combined cycle/HRSG plants has

not been entirely satisfactory. Major failures are still occurring and the locations involved are

basically the same as they were in the 1980s and 1990s. Discussion of the mechanism of FAC

with particular emphasis on fossil and combined cycle/HRSG plants including a) typical

locations of FAC, b) oxides which grow in the areas of interest, c) the cycle chemistry

alternatives and d) the major influences on FAC of turbulence, geometry, mass transfer, and

materials. Because of the importance of FAC failures and the increased levels of corrosion

products when the cycle chemistry is not optimized, it appears of paramount importance for

organizations to consolidate their inspection, predictive, and chemistry approaches into a

company-wide coordinated FAC program in the same way as many do for boiler tube failure

reduction. Unfortunately such FAC programs are not too common across these industries. This

becomes even more important if changes are made to a unit such as material components (feed

water heaters), piping, valves, tees, reducers and cycle chemistry. Each apparently small

change can make a marked difference to FAC locally.

INTRODUCTION

Although there had been a number of early references, it was thought that by the mid 1980s

sufficient understanding of flow-accelerated corrosion (FAC) had been developed. Confidence

was growing in the industry around the world that the parallel research work conducted in

Germany, France and the UK throughout the 1970s for different facets of the nuclear and

steam turbine industries had addressed the major concerns.

By the early 1960s, FAC, then called erosion-corrosion, had already been theoretically

investigated by researchers [1, 2]. It was demonstrated that the fundamental conditions for the

International Journal of Scientific Research in Engineering (IJSRE) Vol. 1 (2), February, 2017

IJSRE Vol. 1 (2), February, 2017 www.ijsre.in Page 28

development of FAC were processes occurring in the laminar boundary layer on the surface of

the metal. The anodic iron dissolution in water results in the formation of Fe+2

ions and

electrons reacting with a respective cathodic reaction. Normally iron dissolution was

considered as a self-inhibiting process where the pH in the laminar boundary layer increases

and, finally, Fe (OH)2 precipitates after reaching its solubility limit. In this way, the iron

dissolution is markedly impeded. Although a ferrous hydroxide layer already inhibits the iron

dissolution to some extent, the optimum conditions are reached only after this layer is

converted to a magnetite cover layer in a series of condensation reactions.

By the early 1980s there was thought to be consensus on the understanding of FAC along the

following lines [3-8]. Magnetite is the oxide which grows on carbon steel surfaces in the feed

water up to about 280 to 300 °C (536 to 572 °F) under low oxygen. Under most operating

scenarios with laminar flow (thicker fluid flow boundary layer) the oxide is protective where

its growth is usually exactly balanced by its dissolution of mainly ferrous ions into the flowing

water or steam/water mixture. Depending on the temperature its thickness may reach 15–25

µm but at temperatures below about 150 °C (302 °F) it can be very thin. Magnetite growth is

controlled by the local cycle chemistries, which at that time were not defined as clearly as

today. Wherever turbulent flow conditions exist as a result of local geometries, the dissolved

ferrous ions are more rapidly removed from the surface. This process is balanced by an exact

growth of more magnetite on the carbon steel surface. This faster oxide removal equates to a

faster overall corrosion process (FAC) and thinner remaining magnetite on the surface (can be

as thin as a few angstrom or equivalent to an interference film). FAC only occurs in water and

water/steam mixtures and not in dry steam, and there is no mechanical damage to the metal as

in liquid droplet erosion and cavitation.

No serious damage had been reported during the 1970s, and perhaps the seriousness of the

first major case of single-phase FAC at the Navajo fossil plant in Arizona in 1982 had not

been identified or recognized [9- 13]. This level of confidence was removed in 1986 when

four workers were killed at the Surry nuclear plant. This event led to a coordinated effort to

ensure that FAC was indeed understood at the power plant level and to a large amount of

research and organizational work to develop a coordinated approach of inspection and non-

destructive evaluation (NDE). This eventually led to the development of a number of

sophisticated FAC codes or models. Since 1986, there have been three more incidents where

plant workers have unfortunately been killed as a result of single-phase FAC: Pleasant Prairie

(1995), Mihama (2004) and most recently at Iatan (2007).

During this time period a new generating source, the combined cycle/ heat recovery steam

generator (HRSG), has emerged into the generating industry in enormous numbers all around

the world [14-18]. Very quickly FAC became the number one availability problem in the

HRSG with large numbers of single- and two-phase FAC failures occurring. Table 1 shows the

locations of FAC in conventional fossil plants [19].


IJSRE Vol. 1 (2), February, 2017 www.ijsre.in 9

Figure 1 shows an FAC failure in a carbon steel economizer inlet header tube [20]. The

surface looks like an orange peel, which is the typical appearance of single-phase FAC. There

is no evidence of any mechanical (erosive) damage and the loss of wall thickness is purely a

chemical dissolution phenomenon. In some areas where the FAC rate is not as fast, there is the

appearance of distinct pit like features, but upon further investigation it is clear that these

features have some directionality. Figure 1 also shows a reducer associated with a level control

valve which shows all the same features. Figure 2 shows a detail of the visual appearance of

FAC. Where the FAC is slower and very little wall loss has occurred (towards the lower right

of Figure 2A) a series or "strings" of these "pit-like" features are clearly evident on the surface.

These have been described variously to have a "chevron" or "horseshoe" appearance with the

tip pointing in the direction of flow.

These chevrons are due to the vectors of turbulent flow touching the surface of the component

or surface oxide causing increased dissolution of the oxide at that point because the increased

mass transfer assists the removal of the magnetite. In these areas there is very little oxide

(magnetite) remaining on the surface and if a metallurgical cross-section is prepared, then the

oxide is usually very thin (can be a few µm). Figure 2B shows almost exactly the same

features of single-phase FAC on the surface of a low pressure (LP) HRSG evaporator tube to

illustrate that the key features will be visible wherever single-phase FAC is occurring.

1 Feedwater heater drains (S, T). Most prevalent area where about 60 % of

organizations record problems

2 Piping around the boiler feed pump (S). Includes desuperheating supply piping

3 Piping to economizer inlet headers (S). Especially associated/near valves and supply

Tees

4 Economizer inlet header tubes (S). Most frequent are usually those nearest to header

supply

5 High pressure (HP) feed water heater tube sheets fabricated in carbon steel (S)

6 Low pressure (LP) feed water heater shells (T). Especially near cascading drain

entries

7 Deaerator shells (T) near to fluid entry (HP cascading drains) piping

8 Reducers on either side of valves

9 Air-cooled condenser (T)

Table 1 - Locations of FAC in conventional fossil plants

* S- Single Phase FAC, T- Two Phase FAC


IJSRE Vol. 1 (2), February, 2017 www.ijsre.in 0

FAC occurance in deaerators, where most of the surface is subjected to single-phase flow and

is generally not subjected to FAC [21]. Two-phase FAC in deaerator vessels is primarily

located near to piping which carries fluids into the deaerator. These might be the high

pressure (HP) cascading drains for example. At each of these locations, there is a difference

in temperature/pressure between the entering fluid and the bulk fluid in the deaerator, and

thus the fluid flashes upon entry into the deaerator. This provides a local two-phase

(turbulent) media which "sprays" against the deaerator surface. Figure-3 shows two typical

areas adjacent to HP cascading drain entry points. The two-phase FAC is delineated by a

surface which is usually black and shiny or even enamel like. It often contains pit-like

markings, which sometimes have the chevron/horseshoe directional appearance as seen with

single-phase FAC. Two-phase FAC with these characteristics can be found in deaerators of

units.



MECHANISM OF FAC IN FOSSIL AND COMBINED CYCLE/HRSG PLANTS

The protective magnetite (Fe3O4) layer on carbon steel "dissolves" in a turbulent stream of

flowing water (single-phase) or wet steam (two-phase) [22]. This process reduces the oxide

layer thickness and leads to a rapid decrease in thickness of the base material until the pipe,

tube or pressure vessel bursts. The FAC process can be very rapid and wall thinning rates

higher than 3 mm per year (0.120 inch per year) have been measured. The rate of metal loss

depends in a very complex way on three main factors: the local cycle chemistry, the material

composition, and the fluid hydrodynamics. The FAC mechanism is discussed here and first

includes a description of the electrochemistry of the basic water chemistries which can be

used in these plants, and then describes the formation and properties of the different oxides

(Fe3O4, Fe2O3 and FeOOH) that can exist on the material surfaces.

The protective oxide which forms on carbon steels in systems up to temperatures just below

300 °C (572 °F) consists almost exclusively of magnetite:

This reaction is considered the sum of two coupled processes. The first process really consists

of three simultaneous reactions: a) a direct reaction (oxidation) occurs between iron and the

reducing water to form soluble ferrous species and hydroxides, b) the ferrous species diffuse

through the porous oxide, and c) the ferrous species dissolve by a reductive process that is

promoted by the presence of hydrogen.

At the anodic site (carbon steel surface), an oxidation process occurs:

At the cathodic site, a reduction process occurs (hydrogen evolution):

In reducing alkalized water, the cathodic reaction:

Combining the anodic and cathodic reactions:

Both ferrous ions and ferrous hydroxide can be obtained according to an equilibrium

reaction:

In the second process magnetite forms through the Schikorr reaction:



Figure 4 shows the growth mechanism and morphology of Fe3O4. In single-phase flow at low

velocities and/or in straight sections of pipe or tubing, the flow is laminar and essentially

parallel to the surface of the metal. In this case the velocity varies from essentially zero in the

fluid boundary layer near to the oxide/water surface to a maximum at the centerline of the

pressure vessel/tube. Under such laminar flow conditions, the second process involves the

transfer of the ferrous ions into the bulk water across a boundary layer. In two-phase laminar

flow there is a slow moving layer of liquid along the surface of the oxide.

The semi-protective magnetite which forms under either of these laminar flow conditions

reaches an equilibrium condition where the oxide doesn't continue to increase in thickness

and is usually limited to a thickness of no more than about < 20 µm depending on

temperature. This is because the layer has lots of oxide hydrates and loose (non-protective)

magnetite particles in the outer layers, and there are essentially equal amounts of dissolution

of ferrous compounds into the flow and oxidation of iron at the metal/oxide interface. At

temperatures below about 150 °C (302 °F), the magnetite formation is very slow in some

cases it is difficult to detect a magnetite layer even after a hundred thousand hours of

operation.

As shown in Figure 5, FAC is simply an extension of the semi-protective process where the

dissolution is accelerated by an increasing flow turbulence of water or water/steam mixture

(decreasing boundary layer thickness) which causes an increased rate of ferrous species to

become incorporated into the flow (increased mass transport) [23]. The FAC mechanism

shows how some flow disturbance (bend, elbow, nozzle, valve, tee, reducer, etc.) introduces

turbulence into the flow, which develops flow vectors against the oxide surface as suggested

by the arrows. This causes a reduction or removal (under severe conditions) of the slow

moving boundary layer in single-phase flow and the liquid film in two-phase flow. In this

case the anodic oxidation and growth of Fe3O4 cannot match the flow-accelerated dissolution

or removal of the oxide (Fe3O4).

At temperatures below about 150 °C (302 °F) with very thin magnetite layers and a thin

laminar boundary layer due to the local flow conditions (turbulence), the ferrous ions are

increasingly transported into the bulk flow; the solubility product of ferrous hydroxide cannot

be reached and FAC occurs. At higher temperatures, magnetite is formed markedly faster.

For this reason, the thin porous magnetite layers are found on the surface. This layer is not

adequately resistant and cannot avoid the transport of ferrous ions into the bulk flow. With

very high FAC rates the oxide can be as thin as an interference film and often is black/shiny

or even has other colorations (green, yellow). The ultimate situation is that the corrosion

(FAC) rate increases and the carbon steel component becomes locally thinner.



Figure 4 - Growth of Magnetite Layer on solid surface

Figure 5 - Schematic of mechanism of FAC

FACTORS AFFECTING FAC

The mechanism of FAC is complex and influenced by three main factors: cycle chemistry,

flow hydrodynamics and the material composition [24].

Cycle Chemistry: All serious cases of FAC have been observed in systems with low oxygen

levels (~1 µg · kg-1

). However, it has been found practically that it is not possible to assess

the likelihood of FAC by separately monitoring the oxygen. With laminar flow the oxygen

will diffuse across the boundary layer.



pH is the second most important chemistry influence on FAC. Although it is the at

temperature pH that is important, it is very rarely measured in fossil and HRSG plants. At

204°C (400 °F) Fe3O4 is about 10 times more soluble in reducing water at a pH of 8.7

compared to 9.6 (Figure 6). There is a linear dependence of FAC on pH between 9.0 and 9.7

due primarily to the solubility change. The pH locally at the site of active FAC will decrease

as FAC proceeds.

Flow Hydrodynamics: While FAC is controlled by the cycle chemistry, it is located by the

complex interactions of the flow hydrodynamics, and specifically by the geometrical features

which create turbulence in the flow and increase mass transport of the soluble ferrous species

from the surface. It has not been found practical or economic however to change the flow

hydrodynamic factors including the temperature of operation.

Flow Velocity: FAC is not directly dependent on flow velocity, and recent computational

analyses confirmed that the mean flow rate is not a good indicator of the FAC process.

Generally FAC shows a rather weak dependence on bulk velocity: FAC rate is increased by

three times when the flow rate is increased from 1.5 to 9 m·s-1

. Also there isn't a threshold or

critical velocity above which FAC begins to accelerate.

Figure 6 - Solubility of magnetite as a function of temperature

Temperature: Some of the original experimental FAC studies illustrated a strong

dependence on temperature with a peak at about 140 °C (284 °F) for FAC. Numerous

subsequent studies have generally shown similar peaks and temperature profiles. Two

examples are shown in Figure 7, where the maximum FAC rate is between 130 and 150 °C

(266 and 302 °F). The temperature of the maximum FAC rate appears to increase with

increasing flow rate or mass transfer. The observation of high FAC rates above 140–150 °C

(284–302 °F) is consistent with the numerous examples of FAC in fossil plant feed water



systems (economizer inlet header piping and tubing). The lower FAC rates at temperatures

above 200 °C (392 °F) reflect that temperature affects changes in other physical and chemical

parameters which influence mass transfer (fluid density, viscosity, etc.).

Mass Transfer: FAC does not often occur in straight pipes or tubes, but is most often

encountered at points of hydrodynamic disturbance. These include elbows, tight bends,

reducer tees, locations downstream of flow control orifices and valves, and even fabrication

discontinuities. The geometric enhancement of these features increases turbulence and mass

transfer. Mass transfer has been expressed in terms of the correlation:

Sh= a*Rex*Sc

y

where the Sherwood number, Sh=Kd/D, the Reynolds number, Re=Vd/ˠ , the Schmidt

number, Sc= ˠ/D, K is the mass transfer coefficient, D is the diffusion coefficient of the

relevant species, d the characteristic dimension of the geometry, V is the flow velocity, and ˠ

is the kinematic viscosity. Various persons over the last 30–40 years have investigated the

effects of mass transfer on FAC. Laboratory studies were conducted. Bates for instance

showed a cubic relation at 150 °C (302 °F) between FAC of carbon steel and the mass

transfer coefficient in PWR type water at a pH of 9.05 over a wide range of flow rates.

Further studies showed that the exponent was temperature dependent and decreased to around

1.0 at about 90 °C (194 °F).

Material Composition: Very extensive worldwide research over 25–30 years has shown that

small additions of chromium to carbon steel will markedly reduce any FAC. Up to about 25

times improvement in FAC rates can be achieved by using 1.0 or 1.25 % chromium alloys.

Figure 7 Solubility of magnetite as a function of temperature at various

ammonia concentrations



Figure 8 - Temperature dependence of FAC under different flow

Figure 9 - The effect of chromium on the rate of single-phase FAC



CONCLUSION

Flow-accelerated corrosion (FAC) has been researched for over 40 years, and scientifically

all the major influences are well recognized. However, the application of this science and

understanding to fossil and combined cycle/HRSG plants has not been entirely satisfactory.

Major failures are still occurring and the locations involved are basically the same as they

were in the 1980s and 1990s. Because of the importance of FAC failures and the increased

levels of corrosion products when the cycle chemistry is not optimized, it appears of

paramount importance for organizations to consolidate their inspection, predictive, and

chemistry approaches into a company-wide coordinated FAC program in the same way as

many do for boiler tube failure reduction. Unfortunately such FAC programs are not too

common across these industries. This becomes even more important if changes are made to a

unit such as material components (feed water heaters), piping, valves, tees, reducers and

cycle chemistry. Each apparently small change can make a marked difference to FAC locally.

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