Accelerated test based on EIS to predict buried steel pipe

Corrosion & Prevention 2013 Paper 138 - Page 1

ACCELERATED TEST BASED ON EIS TO PREDICT BURIED STEEL PIPE CORROSION

N.B. Goodman1, T.H. Muster2, P. Davis1, S. Gould1, T.G. Harvey2, D. Marney1 1CSIRO Land and Water, Melbourne, Victoria

2

CSIRO Materials Science & Engineering, Melbourne, Victoria

SUMMARY: Steel is the most widely used material for pipelines from water supply storages to reticulation networks. Predicting its service-life performance is therefore critical to avoid catastrophic failure and interruptions to supply. Current lifetime prediction models rely on the development of accelerated metal-soil electrochemical tests and correlation of data with in-service performance. Existing methods are limited because they are carried out under pre-determined moisture levels and provide little or no detail about the depth dependence of corrosion performance.

In this work an experimental test column has been used for electrochemical testing of mild steel coupons at various sample depths under specific conditions of moisture content. The final column design ensured that reliable contact was maintained between the metal surface and the soil. The corrosion rate of coupons at the top of the column varies considerably with drying and three distinct regions of corrosivity can be mapped; highly corroding, not corroding and a third, intermediate, transition region which is defined by slope (k). The corrosion rate of coupons at the bottom of the column is less dependent on moisture loss and is regulated by oxygen diffusion through the above soil. At a critical moisture content oxygen diffusion causes acceleration of corrosion processes at the bottom of the column.

Corrosion rate measurements from this investigation may be integrated into existing models, to assist better predictions of asset failure, initially for unprotected steel pipes, but possibly extending to coated and cathodically protected pipes

Keywords: Mild steel, Accelerated, Pipeline, Impedance, Polarisation resistance.

1. INTRODUCTION

Water supply networks are comprised of pipes of varying diameter and materials. Steel pipe has been widely used in the USA since the 1850s (AWWA 2004) and but was only introduced into Australia towards the end of the 1800s, with the majority of large diameter steel pipes (i.e., greater than 600 mm diameter) only having been installed since the 1940s. The corrosion of steel components in soils is of interest to several industries; water and electricity services, oil and gas, and building and construction. The corrosion and associated maintenance costs are a current and future burden on nations with aging networks. Recent studies have determined the cost of corrosion for the urban water industry to be in the range $60-120 per capita per annum (Moore 2010). Today steel remains the most widely used material for large diameter pipelines that transfer water from large reservoir/water supply sources to storages for reticulation, due to their ease of manufacture and robust mechanical properties,. Although this represents a small length of the overall system, it is a critical component for the industry. For instance, steel pipes constitute about 4% of the total length of US drinking water mains, yet their use for large diameter and critical trunk mains suggests that this accounts for 20% of network asset value and therefore warrants ongoing management to avoid catastrophic failure and network down-time.

Corrosion & Prevention 2013 Paper ### - Page 2

The majority of steel pipes are buried in soil to a depth exceeding 600 mm, thus making them difficult to inspect and to monitor for potential degradation and/or breakages. In the broadest sense, steel pipe lifetime may be considered to be dependent upon a combination of external loads, internal pressures, and the corrosion rate of the pipe material. Pipes are usually engineered to cope with internal pressure fluctuations, and their design allows for a range of external load variations, however the corrosion rate is an estimate at best. Steel pipes will undergo both generalized and pitting corrosion and require protection both internally and externally (Sancy et al., 2010). Most also have an internal cement mortar lining which provides a passivating barrier layer against corrosion, and a smooth surface that assists in the efficient pumping of water. The majority of case studies, however, have shown that that external corrosion can be significantly more severe than interior corrosion (Rajani and Kleiner, 2003). Previous literature reviews have highlighted that the major variables influencing buried steel pipe corrosion are the soil resistivity and pH, with characteristics such as aeration, wettability, particle size and structure all having secondary influences (Marney and Cole, 2012). The biggest challenge for predicting material degradation within soils is a lack of understanding of how to connect the conditions generated within the soils to their impact on physical degradation processes such as corrosion and deterioration of coatings. For instance, data compiled by Jiang et al. (2009) suggests that the corrosion rate of steel has a complex relationship with soil moisture; where corrosion occurs twice as fast at 60% moisture than under moisture saturated conditions but is significantly decreased where moisture levels drop below 20-30%. Whilst such data is useful, the magnitude of corrosion also changes with soil pH and conductivity, and the depth in which the sample is buried. Therefore, appropriate tests need to monitor the following factors for a given soil type:

1. Soil drying properties and corresponding corrosion rates (at varying moisture content) 2. Knowledge of soil pH and conductivity 3. Corrosion rates as a function of soil depth.

Typically corrosion tests (i.e. ASTM G162) carried out in soils involve the burial of a metal sample in a soil that is saturated with an electrolyte that may be either deionised (DI) or distilled water, or a standard corrosivity solution such as 0.1M NaCl. The tests are run for various periods of time and the corrosion damage is assessed These measurements are then used to extrapolate to long-term predictions of lifetime. Problems with this approach include that:

1. Corrosion is generally tested in terms of mass loss, so testing periods need to be long enough for measurable damage to occur.

2. Exposure of the cut edges of samples artificially increases the measured mass loss. 3. Tests carried out in shallow containers will be exposed to higher levels of oxygen not be present at greater

depths. 4. Soils are usually saturated during testing, which neglects the influence of soil wetting and drainage

characteristics. This paper details an accelerated method for the evaluation of corrosion induced by different soil types at different depths, and outlines a scale of corrosivity that takes into account the wetting and drying properties of the soil. Table 1 demonstrates the likely relationships expected between the soil properties (resistivity and rate of water drainage) and the resultant impacts on the corrosion of steel pipes.

Table 1. Relationships between corrosion rates, soil properties and water table properties, and their importance for pipe-moisture scenarios.

Soil Properties Water table Properties

Scenario Corrosion Rate Soil Resistivity

Rate Water Drainage

Water table Depth (m)

Rate of Water table Movement

Pipe Above Water Table

Assumed to be minimal

Inherent corrosivity of soil

Retention of moisture (background corrosion)

> 5 m

Pipe Below Water Table

Estimated from saturated soil measurement


< 1m


Movement in Water Table

Predicted enhanced rate – to be determined


Estimate length of time for active corrosion

1 m < Depth < 5 m

Estimate length of time for active corrosion

It is our view that a greater understanding is required regarding what correlations, if any, there are between the corrosion rate and the moisture content for individual soils. Existing measurements are reliant on ‘wilt point’ or some random moisture content (Scherer et al., 2007; Dafter et al., 2012) but specific issues that need to be taken into account include:

• water content dependence on soil, climate, water table depth etc. • the limited understanding of the depth dependence of pipe corrosion. • few experimental studies take into account the soil compression that is experienced by pipes in-service.

In developing this approach we report here on progress toward an accelerated test for measuring steel corrosion in soil and correlated laboratory data with the corrosion of exhumed pipes from the field.

2. EXPERIMENTAL

2.1 Metal coupon preparation Four mild steel test coupons (75 x 30 x 3 mm) were used as two working electrode and counter electrode pairs at the top and at the bottom of each column. A 2 mm deep M3 screw thread was tapped into the back of each coupon to allow electrical attachment to the coupon. One M3 screw (4 mm) secured a ring connector crimped onto one end of each electrical wire (500 mm), while the other was used to connect a Gamry multiplexer / potentiostat. Once connected the entire back surface and edges of each coupon were painted with neutral cure silicon sealant to prevent moisture contact. Before assembly of the electrochemical cell, the surface of each coupon was grit-blasted, rinsed with ethanol then by DI water and finally dried under a stream of compressed nitrogen.

2.2 Field soil description and physiochemical data Four soil types were used; pure bentonite (Sigma-Aldrich: 285234), fine sand (Sigma-Aldrich: 274739) and two clay soils taken from a field site in Braybrook, Victoria. Table 2 provides data from steel pipe samples interrogated in the field site from an in service corroding water main and were evaluated in terms of both average and maximum corrosion rate. Soil samples were removed from the bottom and sides of the water pipe and assessed to determine its moisture content, redox potential, resistivity, sulphate and chloride content. The corrosion damage to exhumed steel pipes was tested using an ultrasonic thickness tester, randomly taking the average of 10 measurements over a 6 x 6 grid from a 300 mm x 300 mm area at 3, 6, 9 and 12 o’clock positions of the pipe cross-section.

Table 2. Field assessment data of soil samples and steel pipes from the field site at Braybrook.

Soil

Sample

Pipe installation

year

Moisture content

(%)

Redox potential

(mV) pH Resistivity

(Ω-cm) [SO4

2- [Cl] (ppm)

- Average corr rate

(mm/year)

] (ppm)

Max corr rate

(mm/year)

Field West 1926 23.4 161 7.8 5150 <50 720 0.005 0.055

Field Bottom 1926 31.1 143 9.4 4200 <50 450 0.007 0.083

2.3 Electrochemical cell assembly A sketch of the experimental test column is shown in Figure 1. The design of the column allows greater control of soil moisture content, wetting and compaction. It also enables two ‘pipe’ depths below ground level to be simulated under laboratory conditions (100mm and 400mm).

RainEvaporation

Drainage


Figure 1. Experimental test column

Each experimental cell consisted of an outer tube comprised of a 500 mm length of 65 mm diameter PVC tubing with a 65 mm PVC end cap (Figure 2). A small section (10mm x 10mm) of geotextile fabric was placed at the bottom of each column to prevent clogging of a small drainage valve, which was screwed into a tapped thread and secured with Teflon tape. Two sample support racks were manufactured from 50 mm diameter PVC tubing, 400 mm long (Figure 3). The supports had sections cut out to accommodate a steel coupon at each end. This allowed consistent and symmetrical positioning of the steel coupons in the soil at two depths; 0 mm & 300mm from the bottom of the column (or 100 mm & 400 mm from the top of the column). The column was assembled with all components (excluding soil) and weighed.

Figure 2. Test columns and electrodes ready to be assembled

Figure 3. Grit blasted electrodes with M3 screw thread tapped into the back to secure electrical connector. The back and edges of each electrode (including electrical connection point) were painted with neutral cure silicon sealant to prevent contact with moisture.

2.4 Instron compression of soil The installation of buried steel pipes in Australia specifies that soil cover around pipes must achieve specific minimum compaction levels (AS/NZS 2566, 2002). To enable reproducible packing and contact between soil and metal electrodes a suitable compression force was calculated from literature values of soil compression with incremental force loadings, as determined by one-dimensional oedometer compression tests (ASTM D2435). Testing showed that, for both sand and clay soils, the maximum compression is achieved using forces of 16 kg/cm2, however, over 90% of this compression could be


achieved using a force of 5 kg/cm2

In preparation for packing the experimental test columns, bentonite (powder) was prepared by wetting with deionised water to obtain a final moisture content similar to that of the field samples. Care was taken to avoid over-wetting the soil as this made Instron compression very difficult. To improve workability of the field samples containing clay, these were also partially wet with DI water.

(Lambe and Whitman, 1969). The lower force was found to be sufficient to ensure reproducible compaction.

Each column was packed with soil using an Instron load frame at 5kg/cm2, as outlined above (Figure 4). The cross sectional area for each PVC column was approximately 27cm2

which corresponds to a 135 kg load over this area (1350 N) to compress the soil using nylon rods to make contact between the soil and the Instron. The columns were progressively packed in three or four stages, applying pressure with Instron at a rate of approximately 0.5 cm/min. Once the required load was reached, it was maintained for 5 min before being released. During compression, the drainage tap was left open to allow moisture to be released from the column; also any supernatant liquid was decanted. After initial compression trials, it was found that cap was required on the column during compression to prevent spillage of soil (Figure 5).

Figure 4. Instron with column filled with clay being held under compression for 5 mins at 1350Nm-2

Figure 5. Image of the cap used to prevent spillage; note the provision for wires connected to the working and counter electrodes.

.

2.5 Determine the gravimetric moisture content (GMC) The GMC was determined for the soil being tested at the beginning of the experiment and also through the experiment as the soil in the column dried out. The change in GMC (gravimetric moisture content) for the soil in each column was calculated based on periodic mass loss measurements. The procedure for determining gravimetric moisture content of soil and rock by mass is described in ASTM D2216-98. In this trial approximately 200g of the soil mix was compressed in the Instron as described earlier. The sample was then place in a 103 degree oven for 12 hours and the GMC µ determined by:

µ =𝑚𝑤

𝑚𝑡

Where mw is the mass of water and mt

is the bulk mass.

An understanding of the relationship between GMC and the changes in the solution resistance of the soil is being developed with the expectation that this will ultimately provide a means of predicting the likely polarisation resistance (corrosion rate) for particular soil moisture content. It is expected that threshold value of GMC will be reached where Rs has increased to a point such that the corrosion rate is negligible.

2.6 Electrochemical Impedance Spectroscopy A Gamry multiplexer was used as an electrochemical interface for each mild steel sample pair, allowing an individual two-electrode (working and counter/reference) measurement to be made for each position in the test column. Electrochemical testing was initiated two days after soil packing had been completed; to allow drainage of the column. At the start of the experiment the tap was closed off to prevent further drainage and loss of moisture from the column would be via evaporation from the top of the column. A 60s monitoring of the open-circuit potential (OCP) of the working electrode was completed to check for continuity and to ensure that there were no connection errors with the cell. This was followed by a


potentiodynamic impedance scan by the application of a ± 10 mV sinusoidal potential (relative to the OCP of the working electrode) across a frequency range of 65 kHz to 0.001 Hz. A minimum of 5 cycles was used for data collection at each frequency. The measurement was repeated for each position in the column every 24 hours, starting at the bottom position. The test was continued until such time that electrochemical data became meaningless due to the presence of high soil resistivity (in the order of 10,000 Ω). Impedance data from the scans was fitted to a Randles cell model and the predicted values for Rs (conductivity) and Rp (reciprocal Corrosion rate) extracted and plotted (Section 3.0) Figure 6.

Figure 6. “Randles-cell” equivalent circuit of data: (A) Nyquist plot of impedance demonstrating the ability to measure both conductivity and corrosion rate for each mild steel couple.

2.7 Steel Pipe and Soil sampling procedure The following procedure is intended to be undertaken at each sampling location along the pipe length.

2.8 Soil sampling Soil sampling was undertaken at four positions (top, bottom and each side) around the pipe barrel as shown in Figure 7. A 500g sample of soil was collected at each position using a spade. Soil samples were collected as close to the pipe as possible – adjacent to the points of pipe sampling/ measurement. Samples were numbered using the following scheme; Sample location (e.g. A) sample position (e.g. 1).

Figure 7. Soil and pipe sampling positions

2.9 Pipe sampling/measurement The measurement and/or sampling of steel pipe was undertaken at four positions around the pipe barrel – adjacent to the points of soil sampling.

Pipe sampling required an area of 300 mm x 300 mm for ultrasonic and manual pipe wall thickness measurement to be undertaken in the lab. Where the exhumation of the pipe section was not possible, ultrasonic thickness measurement (only) were undertaken on the accessible portion(s) of the pipe (DN 1200).

Z (real) Ω

Z (im

agni

nary

) Ω

Conductivity Corrosion rate


Where the exhumation of the pipe section was possible the crown and sampling direction was marked prior to exhumation as shown in Figure 8. To prevent damage to the pipe spray paint was used for marking. Figure 8 indicates the type of marking made on the crown of the pipe and direction in which sample position numbers increase. It was assumed that exhumation would be of a pipe length (full barrel) rather than the removal of sections of the pipe barrel. Exhumation of a pipe length of at least 300 mm was required for measurement purposes.

Note: The critical factor in the marking of the pipe sample was that the soil samples could be correctly associated with the corresponding pipe sample. The marking scheme provided here can be viewed as a suggestion. An alternate approach would be to mark A1, A2, A3 and A4 on the pipe itself, noting that the crown and one side of the pipe should be marked prior to exhumation.

Figure 8. Pipe marking scheme prior to exhumation

Pipe measurements were be undertaken at each position of a 6 x 6 grid within a of 300 mm x 300 mm sampling area, as shown in Figure 9. Where sections of pipe are to be cut from the barrel they measured approximately 300 mm x 300 mm. Removal of pipe sections from the pipe barrel is not a requirement. Measurements following pipe exhumation can be undertaken at a location other than the works site should this be required.

2.9.1 Measurement procedure At each measurement location the surface of the pipe was cleaned to remove all debris exposing the uncorroded steel. This was done using a scrapper and soft wire brush.

A PVC template was used to identify the locations for measurement at each position; this was secured to the pipe surface using magnets following cleaning. At least one magnet was used at each corner of the template to ensure secure contact was made. The 6 x 6 grid was then marked on the pipe surface using a paint marker.

The remaining thickness of the steel was then measured using the ultrasonic thickness (UT) gauge. Several measurements were made within each grid location to identify the minimum remaining wall thickness. Where external pitting was observed (and is too small for the UT probe to be placed at the bottom of the pit) a pit depth gauge was used. The pit depth gauge was also used to measure the maximum pit depth.

Where the pit depth gauge was used an additional 10 UT measurements were made to determine the average steel thickness. These measurements were made within the area of the template (the full extent of the template and not specifically the grid) on areas of nominally un-corroded steel. The average steel thickness was used with each maximum pit depth measurement to determine the minimum remaining wall thickness.


Figure 9. Pipe wall thickness measurement grid (shown on pipe wall)

3. RESULTS

Four experimental columns, each containing 4 mild steel coupons were filled with moist soil and compressed according to the method described above. The gravimetric moisture content for each soil was determined (Table 3). This data shows that bentonite is by far the wettest soil used and that sand, unsurprisingly, retained very little moisture.

Table 3. Gravimetric moisture content (GMC) of the soils tested

Soil Description Gravimetric moisture content

Description of soil

Bentonite 64.8% Pure clay (Sigma Aldrich)

Field Bottom 31.6% Field sample bottom of pipe

Field West 28.4% Field sample west of pipe

Sand 3.7% Pure sand (Sigma Aldrich)

Darwin 13.5% Field sample

To illustrate the trends in Rp and Rs observed in the clay soils test, modelled results from the field B test column are presented for the duration of a 200 day trial (Figure 10). The left hand axis represents the solution resistance and the right hand axis the polarisation resistance. The initial solution resistance measured at both the top and bottom of the column was almost identical; bottom 46Ω and top 34Ω. As drying commenced the solution resistance remained relatively unchanged in both cases for a period of 35 days.

Trends in Rs indicate that the Rs at the top increased steadily (from day 35) until a resistive soil environment developed (>5000Ω) around day 147. The Rs at the bottom remained relatively unchanged throughout the test (56 -90Ω) suggesting that the moisture content at the bottom of the column did not change (GMC will be similar to starting conditions).

Examining the polarisation resistance (Rp) data, it can be seen that at the top of the column, a trend of increasing Rp closely followed the observations made earlier for Rs i.e. a steady increase to a threshold value (10,000Ω) at day 147, at this point the corrosion rate is very low.

At the bottom of the column Rp gradually increased and peaked after day 133 to a value of 1507Ω, after which Rp was observed to drop quickly to approximately 300Ω. This trend may be explained by oxygen diffusion to the metal coupons. Initially, at the bottom of the column saturated conditions slowed oxygen diffusion - inhibiting corrosion. As the top of the column dried, oxygen availability increased throughout the column and allowing increased oxygen diffusion to the bottom coupons allowing reactivation of corrosion processes. Another observation which supports the hypothesis that drying increases oxygen availability is the short lag between the observed increases in Rs (and Rp) at the top of the column and the


decrease observed in Rp (increased corrosion rate) at the bottom. The availability of water and oxygen throughout the column is represented on Figure 10, as drying proceeds, the composition of water and air in the soil moves from predominantly water to mainly air.

Figure 10. Modelled Rs (left hand axis) and Rp (right hand axis) for the coupons in the Field B column.

In general for clay soils, a trend of higher polarisation resistance at the top of the column was observed until soil drying created a resistive environment where corrosion was negligible, typically at values >5,000Ω. A ‘cross over point’ where Rp moved from a lower polarisation resistance ‘corrosive’ to a higher polarisation resistance ‘resistive’ was observed to occur approximately 30 to 60 days after the experiment commenced. For bentonite this was after 60 days, for Field B 52 days and Field West after 30 days. An example of this cross over point is shown in Figure 11. Modelled data for the coupons tested in sand indicated that, at shallower depth, a higher polarisation resistance was present (more resistive to corrosion) from the beginning of the test and no cross over point was observed. This cross-over point suggests a possible means for evaluating soils based on their capacity to retain moisture, and would also be a function of soil depth. Upon rewetting of the sand samples, a “lag” period was observed in which Rp values remained high for a approximately 5 days before decreasing to an order of magnitude determined upon initial wetting.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0

1000

2000

3000

4000

5000

6000

0 50 100 150 200 250

Rp (O

hm)

Rs (O

hm)

Days

Field B (Bottom) Rs

Field B (Top) Rs

Field B (Bottom) Rp

Field B (Top) Rp Rs (top) increases with drying to create resistive

Rp (top) coupons effectively not corroding here

Rp (bottom) decreases as oxygen availability improves at the bottom of the column

M

ainl

y W

ater

W

ater

& a

ir

M

ore

air


Figure 11. Polarisation resistance for Bentonite coupons at ambient temperature

At the end of each test (most still in progress), the columns are dismantled and the coupons cleaned (grit blasted) and examined. Figure 12 shows the degradation of the metal coupons removed from Bentonite clay after approximately 200 days (a) top of the column (b) bottom of column. Both sets of coupons show pitting damage, although a more significant amount of deterioration due to corrosion was observed in the coupons from the top, presumably due to the low Rp prior to the cross-over point.

(a) coupons from top of column (b) coupons from bottom of column

Figure 12. Coupons removed from Bentonite clay after approximately 217 days (a) top of the column (b) bottom of column

0

500

1000

1500

2000

2500

3000

0 20 40 60 80 100 120 140

R (Ω

)

Days

Bentonite (Bottom) Rp Bentonite (Top) Rp

Initially lower Rp at shallower depth

Cross over point

Trend towards higher Rp as drying at the top of the column continues


Figure 13. Gravitational moisture content (GMC) of Field W column samples as a function of time.

Observed changes in Rp can be related to moisture loss by an examination of the kinetics of drying in the soil. Figure 13 shows the reduction of GMC to occur in two distinct kinetic regimes, which can potentially be explained by non-specific and specific binding of water within the soil. The first represents the loss of inter particle moisture from the soil, which is released relatively quickly as indicated by the gradient of (i). The second represents moisture that is adsorbed onto the soil surface or traps within microscopic pores; which is more difficult to release (Gregg and Sing, 1982).

The rate of corrosion for steel coupons may be estimated from the accelerated soil tests. The conversion of polarization resistance to corrosion rate involves an estimate of the Stern-Geary constant for steel, typically 2.303

𝑖𝑐𝑜𝑟𝑟 =𝐵𝑅𝑝

Correlation between polarisation resistance and corrosion rate

Where

𝐵 =𝛽𝑎𝛽𝑐

2.3(𝛽𝑎 + 𝛽𝑏)

Both βa and βc can be assumed as 0.025 V/decade (current) and therefore B has the approximate value of 0.0054. As Rp can be determined from the accelerated test, this enables the estimation of both wet and dry current densities (in amperes per square centimetre). For iron (steel) a current of 1 mA/cm2 converts to a corrosion rate of 11.6 mm/yr. The conversion is based upon Faraday’s conversion of electrons per mol and utilises the density of steel (7.88 g/cm3), the mass of Fe per mol (55.85 g/mol) and assume a two-electron oxidation of Fe Fe2+

Table 4. Corrosion rate (icorr) determined from polarization resistance measured in the laboratory

. Table 4 shows data converted from current to mass loss (mm/yr).

Sample

Rp (Ω) Wet

Rp (Ω) dry Wet icorr (A/cm2

Dry i)

corr (A/cm2

) Wet

corrosion rate (mm/yr)

Dry corrosion

rate (mm/yr)

20

21

22

23

24

25

26

27

28

29

30

0 20 40 60 80 100 120 140

GM

C (%

)

Time (days)

(i)

(ii)


Bentonite 75 150 1.153E-05 5.766E-06 0.1338 0.0669 Field B 340 12000 2.544E-06 7.208E-08 0.0295 0.0008 Field W 183 7000 4.727E-06 1.236E-07 0.0548 0.0014

Sand 2000 7303 4.324E-07 1.184E-07 0.0050 0.0014

As a demonstration of the possible means to link the accelerated corrosion test to field measurements, the relative rates of corrosion under both wet and dry conditions could be guided by local knowledge of the soil moisture content. In practice, this will vary depending on climate and local conditions, as well as relative water table levels. If one assumes that over the 86 year service life of the Field W and Field B samples they had a wet:dry ratio of 1 and 3 (Table 5), respectively, the accelerated corrosion test is seen to match the estimated corrosion rates from condition assessment measurements. These results, whilst not necessarily verifying the validity of the approach, do suggest that the protocols used in the accelerated test lead to measurements of the correct magnitude.

Table 5. Comparison of observed and estimated corrosion rates

Sample

Wet

corrosion rate

(mm/yr)

Ratio of Wet:Dry days

Max corr rate

(mm/year)

Estimated average corrosion rate

(mm/yr)

Average corr rate

(mm/YEAR)

Field B 0.0295 3 0.083 0.0094 0.007

Field W 0.0548 1 0.055 0.0068 0.005

4. DISCUSSION

The rate of moisture loss from soils may have a profound effect on the continuation of corrosion. Experimental data shows that, as the soils lose moisture, significant changes in the polarization resistance and the solution resistance can be measured. In general, for coupons exposed to clay soils, initial corrosion rates (as inferred from modelled Rp data) were found to be higher at the top of the column (shallow soil depth), where the environment has a lower moisture content and increased oxygen availability. Corrosion rates were slower and more consistent at the bottom of the column (greater depth), where saturated conditions prevailed. The trend of higher polarisation resistance at the top of the column continued until soil drying created a resistive environment where corrosion was negligible.

The moisture conditions of the soil (GMC) will enable an estimate of the corrosion rate for steel at the specific location. As a demonstration, it was assumed that Field B (bottom) of pipe would be wet for a slightly greater period of time than Field W (side of pipe). Though further work is required to better understand the precise environmental conditions that the Field B and W pipes are exposed to (i.e. rainfall, water table), initial relationships between the accelerated laboratory trial and field are in the correct order of magnitude. For instance, the average corrosion rates may have good agreement. The maximum corrosion rate estimated from field trials has excellent agreement for Field W, whereas Field B, whilst being in the sample order of magnitude, is higher than expected in comparison to the laboratory test.

The corrosion test column allows successful control of moisture retention and good metal-to-soil contact, both of which are key experimental parameters. This design has proved successful and the use of a single vessel containing immersed coupons in compressed soil under a controlled load (Instron) are key features of this design. Some minor optimization will be required in the methodology to improve the adhesive used on the back of each coupon as delamination of this was observed on some coupons taken from the bottom of the column.

Rewetting of the sand column suggests that reactivation of corrosion processes requires longer periods of solution exposure. As the column drained and dried Rp was observed to increase to previously high values. A second rewetting episode led to similar observations, however further investigation of data surrounding wetting and drying events will be carried out to create an understanding of these effects.

5. CONCLUSIONS

1. The test column and Instron sample preparation procedure was successful in allowing excellent soil-metal contact and control of moisture loss.

2. The corrosion rate of coupons at the top of the column varies with drying. Three distinct regions can be mapped, highly corroding, not corroding and a third, intermediate, transition region.


3. The corrosion rate of coupons at the bottom of the column varies with oxygen availability and is less impacted by moisture loss. Although for oxygen to reach samples at the bottom, moisture must first be removed from the soil above

4. The rate of moisture loss from the column can be described mathematically and linked to observed changes in polarization resistance.

5. An estimate of soil GMC and the number of days of exposure to this environment can be combined to give a greater understanding of the expected pipe lifetime. This will be improved through further development of a methodology to evaluate GMC as a function of soil depth.

6. Moisture is lost principally from the top 1/3 of the column (need to pull in data and do a final GMC to verify this).

7. Rewetting of columns indicates short delays before reactivation of corrosion processes.

The approach described promises to compliment condition assessment in providing valuable guidance on assets to maximise service life and to ensure timely maintenance to avoid catastrophic failures.

6. ACKNOWLEDGMENTS

This work was carried out in collaboration with Water Research Foundation project 4318, Long-Term Performance Prediction of Steel Pipe.

7. REFERENCES

AS/NZS 2566.2 (2002) Buried flexible pipelines – Installation. Standards Australia, Sydney, Australia.

ASTM G162-99 (2010) Standard practice for conducting and evaluating laboratory corrosion tests in soil. West Conshohocken, PA, ASTM International.

ASTM D2435/D2435M-11 : Standar Test methods for One Dimensional Consolidation Properties of Soils Using Incremental Loading. West Conshohocken, PA, ASTM International

ASTM D2216-98 : Standard Test Method for laboratory Determination of Water (Moistrue) Content of Soil and Rock by Mass. West Conshohocken, PA, ASTM International

AWWA (2004) Steel pipe: a guide for design and installation. 4th ed. AWWA manual, M11. Denver, CO: American Water Works Association.

Dafter M, Melchers RE, Nicholas DM (2012) Prediction of long term corrosion in soils using electrochemical tests. ACA Corrosion and Prevention, Melbourne, Paper 14, 1-12.

Gregg S and Sing K. Adsorption, surface area and porosity. Academic Press, London, 1982.

Jiang J, Wang J, Wang W and Zhang W (2009) Modeling influence of gas/liquid/solid three-phase boundary zone on cathodic process of soil corrosion. Electrochimica Acta, 54(13) 3623-3629. Lambe T.W and Whitman R.V (1969) Series in Soil Engineering, Soil Mechanics. John Wiley and Sons Inc. ISBN 0 471 51192-7.

Marney D and Cole I (2012) The science of pipe corrosion: A review of the literature on the corrosion of ferrous metals in soils. Corrosion Science 56, 5-16.

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8. AUTHOR DETAILS

Nigel Goodman has a Masters of Environmental Engineering, Melbourne University and a Bachelor of Science (Honours), Melbourne University. He is a research scientist working in CSIRO’s Urban Water Program and his expertise is in corrosion, water and wastewater treatment technologies as well as in developing and demonstrating techniques for salt reduction for industrial and agricultural purposes. He is author / co-author of 2 book chapters, 7 Journal papers, 12 Conference papers and over 50 reports.

Dr Tim Muster leads the Urban Water Technologies activities within CSIRO. He has a background in colloid and surface chemistry. Since joining CSIRO in 2001 he has developed expertise in the development of corrosion management technologies for structural health monitoring applications. Dr Muster has published over 60 refereed journal publications with >500 citations and has an H-index of 14. In 2007 Dr Muster was the recipient of CSIRO Young Scientist John Philip Award and has twice won the Marshall Fordham Best Research Paper at Corrosion & Prevention (2003 and 2005). More recently, Dr Muster was the recipient of a CSIRO Julius Career Award for nutrient recovery from wastewater.

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