justnes-corrosion inhibitors for concrete

Civil and Environmental Engineering

Corrosion inhibitors for concrete

Prof. Harald JustnesSINTEF Concrete

NO-7465 Trondheim, Norway


Content

Introduction to corrosion and inhibitors Short review of some recent literature. Calcium nitrate is shown to be an inhibitor against chloride

induced corrosion of rebars in concrete, similar to the well-known calcium nitrite.

Demonstrated by pictures of rebars in concrete exposed for chlorides and theory after LPR experiments on mild steel inserts in mortar.

Presentation of extended research program for laboratory and field comparison of calcium nitrate and nitrite performance as inhibitors.


Bridge over troubled waters….

Chloride induced corrosion of rebars is perhaps the most common degradation mechanism world wide.

This bridge had to be rebuilt due to heavy corrosion after 25 years


Close-up of concrete element of bridge

Severe situation since the pre-stressed cables also were corroded. The bridge was demolished and a new one built Could inhibitors have prevented this situation?


Definition of corrosion inhibitor

A corrosion inhibitor is defined as a compound that delays the initiation or propagation of corrosion.

An inhibitor does not prohibit corrosion from occurring.

Some people like to use the term “corrosion retarder” instead.


Corrosion of rebar

Oxygen reduction to hydroxyl ions at cathode; O2 + 2 H2O +4 e- = 4 OH-

Iron oxidized first to ferrous hydroxide at the anode; Fe(s) + 2 OH- = Fe(OH)2 + 2 e-

Fe(OH)2 can diffuse away as complex ion, Fe(OH)4-xClx2- and oxidize 4 Fe(OH)2 + O2 + 2 H2O = 4 Fe(OH)3

Ferric hydroxide has 2-4x higher volume than the metal cracking


Cathodic and anodic inhibitors

Cathodic inhibitors delays the reduction of oxygen by complex formation at the surface of iron/iron oxide; Aminoalcohols like DMEA; (CH3)2N-CH2CH2OH ”Organic” corrosion inhibitors (OCI) as blend of aminoalcohol

(active) and ester emulsion (passive) ”Migrating” corrosion inhibitors (MCI) as substituted ammonium

benzoates (liberates amine by alkalis in situe)

Anodic inhibitors oxidizes ferrous hydroxide to ferric hydroxide at the steel and strengthen passive layer; Inorganic salts of nitrites, NO2

-, e.g. Ca(NO2)2 Inorganic salts of nitrates, NO3

-, e.g. Ca(NO3)2


3 potential applications of inhibitors

1. In patch work repair

2. Applied on the surface for migration to the rebar (MCI)

3. In the bulk concrete from construction

The latter method seems to function best according to literature data.


Long-term effect of inhibitors in patchwork repairNACE Corrosion 2003, San Diego, March 19, paper 03286by Ali Akbar Sohanghpurwala, Concorr inc.

Tested calcium nitrite and 2 different amines (MCI). Evaluations 2, 5 and 6 years after repair. 3 constructions with sufficient chloride to initiate corrosion

showed that corrosion initiated and propagated in spite of the presence of inhibitors.

No difference relative to reference was observed.


Corrosion Inhibitors in Concrete-1

NACE Corrosion 2003, San Diego, March 19, paper 03288by R.J. Kessler, R.G. Powers & W.D. Cerlanek

Tested calcium nitrite, ORG1 and ORG2 Exposed in 3% NaCl in outdoor tank after 90 days curing Conclusion:

Calcium nitrite was the only inhibitor to show significant increase in TCI and TTF in non-pozzolanic concrete (w/c = 0.41)

20% FA lead to >3x TCI and TTF relative to control 8% SF lead to >6x TCI and TTF relative to control

TCI = time to corrosion initiationTFF = time to failure


Corrosion Inhibitors in Concrete-2

Admixture TCI, -280 mVvs SCE (days)

TTF (days) % failed

None 209 298 100

ORG1 245 418 100

DCI-S 484 571 100

ORG2 168 279 100

20% FA >757 >958 80

8% SF >1793 >1793 0


Accelerated testing of inhibitors

Kondratova, Montes & Bremner, ACI SP192-48, 2000

Concrete with w/c = 0.25, 0.40 or 0.60, 20 mm cover,0.4 mm crack or not, 3 inhibitors; CNI, OCI and MCI.

Exposed to artificial seawater in lab. Using LPR++ Conclusion:

CNI effective in sound and cracked concrete of w/c = 0.40 - 0.60. OCI effective in sound and cracked concrete of w/c = 0.40, but

only in sound concrete at w/c = 0.60 MCI was effective in sound concrete of w/c 0.60 and increased

corrosion rate in crack and also for w/c = 0.40 without crack. Pitting corrosion in cracks for HPC (w/c = 0.25), which seems to be

deeper when inhibitors were present.


Natural marine exposure of concrete

Kondratova, Montes & Bremner, Cement and Concrete Composites, Vol. 25, 2003, p. 483-490

2 inhibitors; CNI (calcium nitrite) and OCI (amine based) w/c = 0.40, 505 kg cement/m3, sound and pre-cracked Treat Island in Bay of Fundy, Maine, 120 freeze/thaw yr Conclusions:

Beneficial with inhibitors in sound concrete In pre-cracked concrete with 20 mm cover, significant localised

corrosion at 1 yr and cracking due to corrosion after 3 yrs 5 OCI more efficient than 25 CNI/m3 uncracked concrete In pre-cracked concrete (0.2 and 0.4 mm), both OCI and CNI was

relatively inefficient


Rebars cast in concrete (SINTEF)

20 mm rebars were cast in the center of concrete cylinders of 100 mm diameter and 200 mm height, leading to a concrete cover of 40 mm

The concrete consisted of ordinary Portland cement (CEM I-42,5-R), natural sand, crushed gravel and tap water to w/c = 0.54.

The concrete was added 0, 2 or 4 % dry calcium nitrate (CNA), Ca(NO3)2, of cement weight.


Chloride exposure Series 1: Intermixed chlorides in the form of 3.2 %

NaCl of cement weight dissolved in water.

Series2: Intruded chlorides after 56 days moist curing by exposure to cyclic drying and wetting by 5% NaCl.

Series 1&2 stored at 38°C and 90% RH with air access and inspected for cracks every 3 months.

3 specimens were made for each of 6 variants.


Reference concrete after 2 years

38ºC, 90% RH storage Chlorides intruded by

drying/wetting cycles after 56 days curing

Upper photo rebar Lower photo imprint of

rebar


Concrete with 2% CNA after 2 years




rebar


Concrete with 4% CNA after 2 years




rebar


3.2 % NaCl intermixed concrete stored 3 years at 90% RH & 38°C


Concrete with 2% CNA & 3.2% NaCl intermixed after 3 years


Concrete with 4% CNA & 3.2% NaCl intermixed after 3 years


Steel from concrete with 3.2% NaCl intermixed after 3 years


Steels from concrete with intruded chlorides after 4 years


Corrosion of mild steel in mortarBase mortar composition:

1.00 parts OPC cement

3.00 parts standard sand

0.20 parts lime stone filler

0.55 parts tap water

Mortars made with both 0 % and 3.86 % CNA.


Experiment mortars with steel

Proceeding the lime water curing for 55 days after 1 day in covered molds:

Mortars with embedded steel were stored at 50% R.H. for 7 days and immersed in 5% NaCl solution for 3 days

The procedure was repeated twice (totally 3) in order to create a high Cl- concentration that would initiate corrosion

The prisms were stored at 90% R.H. until the corrosion rate measurements


Polarization resistance measurements

Corrosion rate evaluated by average corrosion current; icorr < 0.1- 0.2 A/cm2 = negligible corrosion icorr > 0.2 A/cm2 = active corrosion icorr > 1 A/cm2 = important corrosion icorr > 10 A/cm2 = severe corrosion

33Ø6

40

160

40


Corrosion current

After a total of 14 months curing:

without CNA

icorr = 2.8 ± 0.8 A/cm2= important corrosion

(High S.D. due to 1 parallel low; 3.2, 3.2 and 1.9)

with 3.86 % CNA

icorr = 0.54 ± 0.05 A/cm2= active corrosion


Corrosion current

After a total of 25 months curing:

without CNA

icorr = 8 ± 5 A/cm2 = important severe corr.

(High S.D. due to 1 parallel low; 11.7, 10.6, 2.6)

with 3.86 % CNA

icorr = 0.29 ± 0.07 A/cm2= active negligible

Chloride content in mortars determined to 2.5 % Cl- of the cement weight


Conclusion corrosion rate

Effect of 3.86 % CNA addition on Cl- induced average corrosion rate:

Reduced with a factor of 5after 14 months curing

Reduced with a factor of at least 26after 25 months curing

Is nitrate (NO3-) as good a corrosion

inhibitor as nitrite (NO2-)?


Nitrite inhibiting mechanism Sacrificial reduction of nitrite to form FeOOH (or

similar) at the anode, which will stifle iron dissolution:

Red.: NO2- + H2O + e- = NO + 2 OH- E0 = - 0.46 V

Ox.: Fe(OH)2 (s) + OH- = Fe(OH)3 (s) + e- E0 = + 0.56 V

which combine to the total reaction;

Fe(OH)2 (s) + NO2- + H2O =

Fe(OH)3 (s) + NO + OH- E0 = + 0.10 V


Nitrate inhibiting mechanism Nitrate will give an extra reduction step

compared to nitrite;

2 Fe(OH)2 (s) + NO3- + H2O = 2 Fe(OH)3 (s) + NO2

-

E0 = + 0.57 V

This imply that nitrate, NO3- , should be an even

better inhibitor than nitrite, NO2- , since two more

moles of ferrous iron are oxidized to ferric per mole nitrogen oxide added!


Nitrates is less harmful than nitrites

Impure form of calcium nitrate (with some ammonium) is actually used as fertilizers and spread in the fields.

Acute lethal dosage for 50% rats by oral route (LD50) is for Sodium nitrite, NaNO2, 180 mg/kg Sodium nitrate, NaNO3, 1,267 mg/kg Calcium nitrate tetrahydrate, Ca(NO3)·4H2O, 3,900 mg/kg

In addition nitrites are mutagenic and nitrates not.


Extended program nitrate vs nitrite-1 Extended program initiated autumn 2001

Concrete recipe 350 kg OPC/m3, w/c = 0.47

14 concrete mixes for rebars in cylinders in lab/field:

2 types commercial calcium nitrate, commercial calcium nitrite and sodium nitrite

2 dosages calcium nitrate, one dosage others + reference

Mixes with intruded and intermixed (3.2%) sodium chloride


Reference with intermixed chlorides after ~3 years at the roof top (average T = 5C)

Cracked


Rebar of concrete with intermixed chloride

3 years weather exposure of reinforced concrete on roof top. ”Green rust” to the left as ferrous hydroxy-chloro complex in newly opened

specimen with intermixed chlorides (Fe2+ sign of limited O2) After the rebar has been laying 3 days in laboratory air, the ”green rust” had

turned into ordinary red ferric rust by oxidation (Fe2+Fe3+)


Extended program nitrate vs nitrite-2

3 self compacting concrete (w/c = 0.50) reinforced wall elements were cast, instrumented and placed (3 days old) in the tidal zone + “lollipops” for rapid test.

The three mixes were reference (no admixture), 4% calcium nitrate by cement weight and equivalent commercial calcium nitrite (i.e. equal amount molar nitrite as nitrate in mix 2).


“Lollipop” test-1 Concrete recipe w/c = 0.50 with 330 kg OPC Reference (0% admixture) 4% dry calcium nitrate bwoc Equivalent molar dosage of commercial calcium nitrite 100x280 mm cylinders were cast with 20 mm rebar in the

center and 50 mm from bottom, leaving cover of 40 mm. Test is an impressed current test (5V DC) measures time

to failure. Increase in current flow and eventually cracking due to expanding corrosion products.

Brown, R.P. and Kessler, R.J. in Report no. 206, Florida Dept. Transportation Office of Materials and Research. October 1978.


“Lollipop” test- 2 concrete propertiesProperty Reference Nitrate NitriteFreshSlump (mm)Density (kg/m3)Air (vol%)

14024252.1

16023953.1

13524202.3

c, 1 d (MPa) 28.90.1 21.10.2 25.90.6c, 28 d (MPa) 54.50.6 60.30.6 77.30.590 daysc, 90 d (MPa) (kg/m3)dry (kg/m3)cap (vol%)mak (vol%)

66.10.92691423892610.10.71.20.2

70.50.42690523671210.50.31.50.06

84.01.3268842394129.90.31.00.03


“Lollipop” test - 3 Set-up


“Lollipop” test - 4 Example


“Lollipop” test - 5 Experiment

Reference; 32 days to current increase, 2 of 3 cracked at 37 days Nitrite; 32 days to current increase, 2 of 3 rust up along rebar at 33 d Nitrate; small crack at 65 days. Corrosion products seen at 120 days

5 V "Lollipop" testing of rebar corrosion

10,012,014,016,018,020,022,024,0

0 20 40 60 80 100 120Time (days)

Cur

rent

(mA

) pa

ssin

g

ReferenceNitrateNitrite


Wall element concreteProperties Ref. Nitrate NitriteFresh mix

Density (kg/m3)Air (vol%)

2,3845.2

2,3444.8

2,3803.0

1 dayc (MPa) 22.10.2 19.00.3 24.40.3

7 daysc (MPa) 45.30.1 47.80.6 61.90.4

Daysc (MPa)

3556.90.7

3458.30.5

2873.50.6

1 yearc (MPa) (kg/m3)C0, Cl (%)

DCl (10-12m2/s)r (m)

65.20.324125

0.890.077.81.0731

65.51.023972

0.960.019.91.3702

85.42.424572

0.990.076.40.5692


Instrumented reinforcement element

2 types electrodes in 3 level (bottom, middle, top), a special sensor for detection of intruded chlorides, 3 bends for stress corrosion cracking. 2 reinforcement nets with 20 and 50 mm concrete cover.


Special sensor for chloride intrusion

As chlorides intrudes, corrosion will start first on the rebar closest to the surface. By measuring of potential difference between thick, rear rebar parallel to surface against thinner, perpendicular rebars at different depths, an indication of how deep chlorides have penetrated at a given time can be obtained.


Placing instrumented, reinforced concrete wall elements in tidal zone

2 of 3 walls (light grey) at Østmarkneset at low tide. Walls are identical but cast without inhibitor, with 4% calcium nitrate and with equivalent dosage of commercial calcium nitrite, respectively.


Preparing to measure

Wires from all electrodes etc is collected in a box at the top of the wall element to enable quick measurement of all electrodes with a simple multimeter at low tide.


Reinforcement grid with 20 mm cover

Average of 3 electrodes at different levels from bottom

ERE20 average potentials 10-12 vs 16

0

60

120

180

240

300

360

1 10 100 1000Time (days)

Pote

ntia

ls (m

V)

Ref

Nitrate

Nitrite


Reinforcement grid with 50 mm cover

Average of 3 electrodes at different levels from bottom

ERE20 average potentials 10-12 vs 17

-40

20

80

140

200

260

320

380

1 10 100 1000Time (days)

Pote

ntia

ls (m

V)

RefNitrateNitrite


Conclusion-1

Anodic inhibitors seems to function better than cathodic according to literature review.

Calcium nitrate is an anodic inhibitor against chloride induced corrosion of steel in concrete.

It works both for intruded and intermixed chlorides.

3-4% dry calcium nitrate of cement weight seems to be sufficient to obtain good protection of the rebar


Conclusion - 2 The inhibitor mechanism of nitrate (NO3

-) is analogous to the well-known corrosion inhibitor nitrite (NO2

-), but the kinetics may be slower.

According to mechanism, calcium nitrate provides a higher buffer as corrosion inhibitor than calcium nitrite at equimolar dosage.

Calcium nitrate is less harmful, available in larger amounts and cheaper than calcium nitrite.


SINTEF not the only one to claim that nitrate work as inhibitor

Al-Amoudi et al. Cement and Concrete Composites, Vol. 25, May 2003, pp. 439-449

Concrete with w/c = 0.45 and 350 kg c/m3

Exposed to chloride, chloride/sulphate, seawater, brackish water, unwashed aggregate

2 and 4% calcium nitrite by weight of cement 2, 3 and 4% calcium nitrate by weight of cement CI1 inhibitor (organic) in recommended dosage (5 /m3) CI2 inhibitor (organic) in recommended dosage (1.2 kg/m3) Current density by LPR method reported.


Current densities (A/cm2) after 122 days,Al-Amoudi et al

Nitrite NitrateCont. Ref2% 4% 2% 3% 4%

CI1 CI2

Cl-Cl-/SO4

2-

SeaBrackishNo wash

.52

.55

.54

.28

.24

.06

.10

.04

.03

.08

.06

.06

.04

.03

.06

.06

.07

.06

.06

.06

.06

.06

.06

.06

.06

.06

.07

.06

.05

.06

.07

.08

.09

.05

.11

.08

.08

.11

.12

.14.06

justnes-corrosion inhibitors for concrete

Documents

corrosion of rebars

propagation of corrosion

heavy corrosion

presence of inhibitors

term corrosion retarder

environmental engineeringclose

patchwork repairnace

situe anodic inhibitors