Concrinox TM

Using stainless steel in concrete reiforcement structures

- What are stainless steels
- Corrosion resistance
- Ductility
- Toughness
- Behaviour at high and low temperatures
- Wear resistance
- Weldability
- Magnetic properties
- Economic considerations
- Application fields
- Specifications
- Technical bibligraphy

Available products

- Improved bond bars and wire rods
- Mesh and lattice girders
- Ask the expert


CORROSION RESISTANCE


Carbon steel used in rebars is normally protected from corrosion since concrete is an environment subject to high basicity (pH 12-13); Under those conditions iron , a basic element in carbon steels, is under passive conditions.
However, depending on operating conditions, it is not always possible to guarantee the physical and structural integrity of concrete. If a carbonation phenomenon occurs (chemical aggression of carbon dioxide contained in the atmosphere in concrete and in particular in lime mix), a decrease in the pH level can be seen, following the relevant reaction

Ca(OH)2 + CO2 --> CaCO3 + H2O

A decrease in concrete alkalinity permits the development of a broad range of corrosive phenomena in carbon steel rebars through the formation of oxides and/or iron hydroxides (typical rust).





Moreover since in some applications concrete is contaminated generally by salt solutions (chlorides penetrate into concrete), the highlighted problem can become very important as regards the structural resistance.

In some examples shown in picture 1, it is obvious how corrosive phenomena are often linked to the formation of rust which having a specific volume up to 6-8 times greater than the iron bars, causes the splitting, the disintegration and in some cases the cracks of the thin concrete coating. (phenomenon called “spalling”).



Picture 1


In stainless steels, the presence of chromium in huge quantities gives them the capacity to “self passivate” in a spontaneous way when the clean surface is in contact with an external environment, corrosive or oxidative (picture 2).


Drawing showing stainless steel passivation mechanism.
Picture 2 - Drawing showing stainless steel passivation mechanism..


In addition to chromium the elements which contribute to increase corrosion resistance are:
  • Molybdenum
  • Nickel
  • Nitrogen
Carbon acts in an inversely proportional way:the lower its content , the greater its resistance to corrosion.
In concretes with alkaline pH or even neutral ( after carbonation processing) stainless steels do not undergo corrosion (Picture 3).
Corrosion phenomena in stainless steel rebars can be observed only if there is a relevant concentration of chlorides in concrete made porous from carbonation itself. Once such a contraction overcomes a critical threshold then a localised reduction and elimination of the passive oxide layer can be noticed and corrosion then can start, provided that there is an appropriate quantity of oxygen. The phenomenon is better known as pitting.


Use conditions in safety of different steels depending on chlorides concentration and pH
Picture 3 - Use conditions in safety of different steels depending on chlorides concentration and pH


A simple way to evaluate corrosion resistance to pitting of stainless steels is the so-called coefficient or PREN index (Pitting Resistance Equivalence Number) calculated as follows:

PREN = %Cr + 3,3%Mo + 16%N (for austenitic steels)
PREN = %Cr + 3 ,3%Mo + 30%N (for duplex steels)


On the basis of the above formula we can set a classification for common stainless steels such as stainless steel rebars:


AISIEN 10088-1Cogne GradePREN index
304L - austenitic1.4301304HT18
316L - austenitic1.4436316HT25
329 - duplex1.4462329HT35
---- superaustenitic1.4529354/145
Tab. 2 - Classification of Stainless steels used in concrete according to their resistance to pitting


ClassConcrete type and environmental conditionsAggression levelRecommended stainless steel grade
   Common applicationsHigh-level safety structures
1
  • Carbon enriched concrete
  • Environment slightly alkaline
  • Chlorides free
Modest1.4301
(AISI 304L)		1.4301
(AISI 304L)
2
  • Concrete with a normal level of alkalinity and Cl- <1%
  • Slightly Carbon enriched concrete
Rather aggressive1.4301
(AISI 304L)

1.4436
(AISI 316L)		1.4436
(AISI 316L)
3
  • Carbon enriched concrete with Cl- >1%
  • Slightly Carbon enriched concrete with chlorides presence
Very aggressive1.4436
(AISI 316L)		1.4462
(AISI 329)
4
  • Carbon enriched concrete in a high chlorides concentration environment
  • Reinforced concrete Structures without any possibility of entry for controls
  • Hinges connecting separate blocks in reinforced concrete exposed to high Cl- or to highly aggressive environments
Extremely aggressive1.4462
(AISI 329)

1.4529
(354/1)		1.4529
(354/1)
Tab.3 Indications of stainless steels applications according to the different environmental conditions. (Euroinox)