Page 397 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION DAMAGE, DEFECTS, AND FAILURES 375

TABLE 5.9 Corrosion Resistance of Some Steels
Alloy Corrosion Resistance PREN Cost Ratio
Steel Poor – 1.0
Steel epoxy coated Poor – 1.7–2.0
S30400 Good/fair 18 3.8
S30453 Good/fair 21 4.3
S31600 Good/better 25 4.4
S31653 Good/better 27 4.7
Duplex 31803 Very good 36 4.4

5.3.7.4.13 Performance of Stainless Steel Rebar in Concrete Corrosion protection
of steel rebar can be achieved by: (i) selection of corrosion-resistant steel; (ii) use of
coatings; (iii) addition of corrosion inhibitors such as calcium nitrite to concrete mix;
(iv) addition of concrete sealers; (iv) use of membranes; (v) use of thicker concrete
overlay; (vi) cathodic protection.
Some of the reinforcing steel bar alloys are: cold-worked steel; epoxy-coated
steel; 30400 stainless steel; 30453 stainless; 31600 stainless; 31653 stainless; Duplex
S31803. Table 5.9 shows the corrosion resistance of some steels.
Carbon steel and 304 stainless steels were tested in 0.2% and 0.5% chloride and
exposed for 2 years in 80% relative humidity. The carbon steel failed, and the stainless
steel was free from cracking and corrosion. The concrete also remained intact without
cracking. Life-cycle costs for the Öland Bridge in Sweden were calculated in the
case of carbon steel and 304, 316 stainless steels for a time period of 120 years. For
a duration of 18 years, the cost of carbon steel is less than type 304 stainless steel,
which in turn is less than 316 stainless steel. For a life between 18 and 120 years, the
costs of stainless steels are far lesser than carbon steel (19).

5.3.7.4.14 Corrosion of an Oil Storage Tank A 3 mm hole developed at the bottom
of an oil storage tank after 25 months of service. The premature failure of the tank
raised questions about the construction material of the tank as well as the corrosive
species such as water in the contents of the tank.
The top and bottom sections of the tank were subjected to spectrographic analysis.
The analysis showed it to be SAE-AISI 1006 or 1010 steel. The bottom section (70 cm
long and 33 cm wide) was corroded to an extent of 80%. The surface corrosion along
the edge appeared to be smeared and brushed as a result of cutting. The underlying
metal had a blue–gray mill scale.
About 80% of the surface area was corroded. Some areas had deep corrosion that
penetrated past the mill scale and into the metal substrate. Pitting of a severe nature
is present when one moves beyond the mill scale.
Pitting was present at the bottom of the tank. The area with the hole is the most
severely pitted surface. The pits were deep (∼150 μm) with an external diameter of
50 μm.