FIELD OF THE INVENTION
The present invention relates to a corrosion resistant thermo-mechanically
treated reinforcing bar. The invention further relates to a method for producing a
corrosion resistant rebar through alloy design.
BACKGROUND OF THE INVENTION
The major issue currently facing the concrete construction industry is the
corrosion of steel reinforcing subjected to corrosive environment. Corrosion of
steel reinforcing leads to reduction in strength of concrete member, and causes
the deterioration of surrounding concrete, further damaging the concrete
member.
Newly mixed concrete consists primarily of aggregates, cement powder, and
water, the latter two components forming cement paste, which soon harden by a
process called hydration. As the cement paste hardens it binds the aggregate as
a solid matrix which gives concrete its load carrying ability and durability. The
consumption of the mix water in the hydration-hardening reaction leaves
capillary, and pores in the concrete matrix through which atmospheric gasses,
pollutants, and water can penetrate when the concrete is wetted by rain,
condensation, or spray. The retained water within the pores of the concrete
matrix becomes saturated with the chemical components of the cement and
forms a highly alkaline solution, with a nominal pH of ~12.5, depending on the
specific cement powder used.
Bare reinforcing steel is normally passivated in the initial pH of the contained-
water in newly hardened concrete, however, wetting and drying cycles allow for
atmospheric gasses including carbon dioxide and sulfur dioxide to dissolve in the

pore water, and their acidity in solution begins to lower the pH of the pore water.
This process is called carbonation.
Bare reinforcing steel begins to loose its passivation or dormancy as the pH
surrounding the steel passes below about pH 11.5, and rusting of the steel
begins and progresses, the resulting corrosion products take up more space than
the steel consumed and this volume expansion within the constraining, rigid
concrete matrix, is that substantial stresses are exerted on the surrounding
concrete.
In addition to the lowering of the pH of pore space water below the threshold of
passivation of embedded bare steel, chloride ions from the structure
surroundings, also are dissolved in the pore water, and once permeation down to
the steel surface, further act to destroy the passivation of the embedded bare
steel. The time elapsed before the combination of acidic atmospheric
components and chlorides permeate to the embedded steel surface is a function
of the environment, wetting cycles, porosity and composition of the concrete,
and the length and difficulty of traversing the labyrinthine path to the bar
surface. The service life of a concrete structure, degraded by corrosion, is
considered to be the sum of the initiation and propagation periods. The initiation
phase is the time required for sufficient accumulation of aggressive species at
the rebar surface to initiate corrosion. The duration of the propagation phase is
the total time until concrete cracking, caused by formation of corrosion products
from the steel reinforcement, which occupy a larger volume than the parent
steel.
Weathering steel containing small amounts of alloying elements such as
Cu,Cr,Ni,Si, and P has been widely used because of its excellent resistance to
atmospheric corrosion. This is because of an adherent, protective layer formed

on the steel on prolong exposure in atmosphere. However, it does not provide
adequate corrosion protection in oxygen deficient situation such as prevailing in
concrete environment.
To prevent corrosion of rebar of conventional steel bars, an optimum
combination of constructional design a high quality concrete, and a corrosion-
reducing admixtures in the concrete is one of the solutions. Unfortunately, this
hypothetical combination seldom works. Corrosion-reducing admixtures have not
always proven effective in reducing rebar corrosion and no matter how good the
construction design is and how high a quality concrete is used, concrete is highly
prone to cracking and thus exposing the rebar to chlorides and moisture.
Another option to prevent rebar corrosion, is to coat the rebar with an organic
compound that would protect the steel from corrosion. The coating must be
impervious to chloride, moisture, and oxygen. Another requisite property is that
the coating be durable so that it is not damaged during transportation to the
construction site. It must also be economical. The conventional strategy for
suppressing corrosion of carbon steel reinforcement has been to apply organic
coatings. Unfortunately, a critical weakness of the coating is it's susceptibility to
mechanical damage during transport and placement.
Furthermore, epoxy coated rebar(ECR) has been found to be ineffective in
extending rebar lifetimes, compared to uncoated carbon steel, in concrete
constantly exposed to tropical seawater.
Metallic coatings of rebars can also be used. The first kind of metallic coating is
sacrificial while the second is non-sacrificial. For instance, a galvanized (zinc-
coated) bar creates a galvanic cell where the coating is slowly sacrificed (hence
the term 'sacrificial') and thus prevents the bar from corroding. As in ECR, a

sacrificial coating must be continuous to be effective. Unfortunately, the idea of
sacrificial layers and galvanization was not effective for two reasons. Firstly, it
turned out that zinc tolerates a higher chloride level before corroding, so all the
galvanized layer does is delay the onset of corrosion for a short while. Secondly,
it was discovered that once the sacrificial layer does start to corrode, it corrodes
at a much higher rate than the black steel. Sacrificial metallic layers are clearly
not the best option for corrosion protection.
The non-sacrificial metal layers, theoretically work by applying a layer of a
relatively non-reactive (noble) metal, like nickel, to the iron bar. The rebar is
formed by applying a heavy layer of nickel to a hot iron billet before it is hot
rolled. It ultimately consists of three layers: an outside layer of nickel, an inside
core of iron, and a middle diffusion zone made of alloyed nickel and steel, which
provides additional corrosion protection in case of a break in the nickel layer. An
eleven-year testing period showed that this type of a non-sacrificial layer is
extremely effective in slowing (and even in some instances, completely
preventing) the corrosion of rebar, even in a high-chloride marine environment.
Also, nickel-layered steel rebar is considerably more expensive than plain steel
rebar, as nickel is a relatively expensive metal.
New candidate rebars being considered are; 2205 and 2101 duplex steels, 316LN
stainless steel, 316L stainless steel clad (over carbon steel), and MMFX-2 steel
rebar. However, the prohibitive cost makes these metals poor choices for rebar
materials, despite their incredible strength and resistance to corrosion.
The present inventors disclosed an intrinsically corrosion controlled grade TMT
rebars to combat the aggressive marine and industrial environments (Indian
patent no:235949,"A method for producing corrosion resistant steel reinforcing

bars").No protective coating is necessary at the time of using these rebars
because the alloying elements form a protective passive layer on the rebar
surface, which prevents further corrosion of the substrate. The corrosion
resistant alloying elements used for the above rabars consisted of Cu
(0.3/0.4%),P (0.07/0.12%) and 0.2 %maximum chromium by weight.
However,high P in the steel is perceived as impurity. Moreover, high P generates
a lower fracture toughness at subzero temperature.
Steel Authority of India have also been producing corrosion resistant rebars
containing Cu and Cr as alloying addition. However, the weight percentage of Cr
(0.5%) used in the steel is prone to give pitting corrosion in saline atmosphere.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a corrosion resistant
rebars with corrosion resistant index of 1.5 in concrete environment, and yield
strength of 500 MPa.
A further object of the invention is to propose a method of producing a corrosion
resistant rebars with corrosion resistant index of 1.5 in concrete environment,
and yield strength of 500 MPa.
SUMMARY OF THE INVENTION
The objects of the present invention can be achieved by a chemistry controlled
rebar containing C: 0.21-0.24 wt %, Mn: 0.90-1.10 wt %, Si: 0.10-0.15 wt%, P:
not more than 0.035 wt %, S: not more than 0.035 wt %, Cu:0.30-0.40 wt%,Co:
0.20-0.35 wt % and Ca: 0.0020-0.0040 wt%.

The corrosion inhibiting elements Cu-Co-Ca is enabled to form a protective rust
layer in the form of cupric oxide compound as well as Co oxides or Co
oxyhydroxide on the new alloy steel in aqueous condition. The addition of Ca
leads to an increase in the alkalinity of the pore solution of concrete near the
steel rebar surface.
The above described corrosion resistant rebars can be manufactured by a
method comprising the steps of producing a thermo mechanically treated rebars
by hot rolling of steel billets having the finish rolling temperature of 580 to
630°C.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l. Shows tempered martensite structure of a conventional rebar
Fig.2. Shows tempered martensite structure of rebar of the invention
Fig.3 Hardness profile from edge to centre of a conventional rebar
Fig.4. Hardness profile from edge to centre of the rebar of the invention
Fig.5. Atmospheric exposure test for a conventional and the inventive material.
Fig.6. Potentiodynamic polarization test showing Tafel plots for corrosion rate.
Fig.7. Mounted samples of rebars wetted with SPS+3.5% Nacl solution showing
loosely bond rust on a known composition and an adherent rust on the new
chemistry of the inventive rebar.

Fig.8. Weight gained by rebar exposed in humidity chamber in SPS+3.5% NaCI
Solution(pH=9.0by adding few drops of HCI) in dry and weight condition
alternately.
Fig.9. Shows the rust layer formed on known rebar mainly composing hematite
(Fe203).
Fig. 10. Shows the adherent layer of y-FeOOH magemite formed on new
chemistry of rebar according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
Laboratory heats were made with conventional mild steel as well as new
chemistry steel of the invention in an air induction furnace and cast to 25 kg
ingot as shown in Table 1. The ingot was forged in to 35 mm thick plates after
soaking at 1200°C. Samples in 200 mm length were cut off from the forged
plates and finished to 18 mm dia by machining. Samples were heated to 900° C
and soaked for 30 minutes, subsequently quenched in water. The quenched
samples were further tempered at 560° C for 10 minutes to get entire structure
of tempered martensite; followed by air cooling.

The microstructure of the conventional /reference material along with the
inventive material shown in Fig.1 and Fig.2, reveals tempered martensite. The
hardness profile of the above sample measured from edge to centre ( shown in
Fig.3 and 4 respectively) depicts slightly lower hardness in the case of the
inventive material. This may be attributed to the addition of Co as it marginally
reduces the hardenability.
The samples from the inventive materials were subjected to various corrosion
test. Reference material (Tata 1) were also tested for comparison. In the
atmospheric exposure test results, it may be clearly seen in Fig.5 that the
conventional rebar is full of rusting within one month whereas in the inventive
rebars, rusting spots were very few. This shows the superior corrosion resistant
of the inventive material over conventional rebars. Samples were also tested by
potentiodynamic polarization in 3.5% NaCI and cathodic and anodic polarization
curves are shown in Fig.6. It clearly reveals that the inventive rebar is having
more than 2 CRI over conventional rebar. The figure also shows a relationship
between the corrosion rate and corrosion potential. The inventive alloy steel had
more noble corrosion potential and lower corrosion current than conventional
rebar indicating the effectiveness of this steel for use in saline environment.
Rebar samples from the inventive and the conventional steel were mounted on
bakalite and polished to expose the cross section of the rebars. Simulated pore
solution (3.5gm of NaOH,10.5 gm of KOH and 2.1gm of Ca(OH)2 in 1 litre of
demineralized water) similar to concrete alkalinity; was prepared in laboratory
and stored in a beaker. Sodium chloride was added to SPS to have a
concentration of 3.5% NaCI by weight. Few drops of hydrochloric acid was added
to the solution to bring down the pH to 9.0. Everyday, mounted samples were
wetted with few drops of prepared solution and put in to humidity chamber at
35° C at 95% relative humidity for 8 hrs and remaining 16 hrs in air at room

temperature. This cycle was repeated daily till 60 days. At the interval of every
five days, the samples were taken out for removing the loose rust and weighed
(Fig.7). The new material formed a dense and impervious rust layer; very
adherent to the rebar surface and did not show any loose rust. Rust at outer
periphery of the conventional rebar due to crevice corrosion was observed.
Spreading of rust at outer periphery of the prior art rebar was also observed. The
weight gain (mg/cm2 /day) for the conventional vis-a-vis the inventive samples is
shown in Fig. 8. It is quite evident from the curve (wt gain vs exposure time)
that initially the inventive rebars show lesser wt gain compared to the
conventional rebar. However, once the thin adherent layer oxide resistant
elements is formed on the inventive rabars, the curve becomes almost horizontal
after 30 days whereas due to loose rust formation on the conventional rebar, the
weight gain becomes negligible. The scale formed after 30 days was subjected to
Raman spectroscopy for characterization. It may be seen from Fig.9 that rust
layer formed on the conventional rebar are mainly composed of hematite
(Fe203) and magnetite (Fe304) where as in the case of the inventive rebar, a
very adheret layer (Fig. 10) of y-FeOOH called magemite is formed which is
highly desirable for imparting the corrosion resistance. This magemite is flawless
and thin impervious layer working as barrier for ingress of corrosion spices.
The main role of calcium is the formation of complex inclusion containing CaO
and CaS, which can dissolve completely in thin water film condensed on the
rebar surface.The dissolution of complex inclusions containing CaO and CaS
increases the alkalinity of the thin water film. Thus, it is logically presumed that
dissolution of Fe and other alloying elements was reduced by the addition of Ca,
which restrained the formation of rust layer.

WE CLAIM
1. A corrosion resistant thermo-mechanically treated reinforcing bar,
comprising C: 0.21-0.24 wt %, Mn: 0.90-1.10 wt %, Si: 0.10-0.15 wt%, P:
0.035 wt %, S: 0.035 wt %, Cu:0.30-0.40 wt%,Co: 0.20-0.35 wt % and
Ca: 0.0020-0.0040 wt%, wherein the corrosion resistant index (CRI) of
the rebar with tempered martensite structure and containing the alloying
elements is at least 1.5, and wherein the yield strength of the bar is
maintained at least at 500 MPa.
2. The reinforcing bar as claimed in claim 1, wherein the corrosion inhibiting
elements cu-co-ca is enabled to form a rust-protective layer in aqueous
condition.
3. A method of producing a corrosion resistant thermo-mechanically treated
reinforcing bar, comprising:
providing a steel billet of composition as claimed in claim 1; and
hot rolling the steel billet at a finish rolling temperature between 560°C
to 630°C to achieve a corrosion resistant index (CRI) of 1.5 in concrete
environment, and an yield strength of 500 MPa.

The invention provides a corrosion resistant thermomechanically treated
reinforcing bar based on addition of corrosion inhibiting elements to a selected
alloy chemistry. The rebar containing C: 0.21-0.24 wt %, Mn: 0.90-1.10 wt %,
Si: 0.10-0.15 wt%, P: 0.035 wt %, S: 0.035 wt %, Cu:0.30-0.40 wt%,Co: 0.20-
0.35 wt % and Ca: 0.0020-0.0040 wt% having tempered martensite structure.
The inventive reinforced bars provide 1.5 times superior corrosion resistance
over conventionally produced mild steel reinforcing bars.