CORROSION: Intergranular Corrosion

Like other common materials, metals have a visible grain structure when they are viewed under magnification. Rapid corrosive attack of immediately adjacent grain boundaries with little or no attack of the grains is called Intergranular Corrosion. 

Rapid attack at the grain boundaries can result in grains “dropping” or falling out of the metal surface resulting in the disintegration of the steel. Figure 1 shows the appearance of a surface where this is occurring.  In practical application, the loss of cross section thickness and the introduction of cracks can have severe consequences for applications like pressure containment. 

Figure 1: Intergranular corrosion grain boundary attack and dropped grainsPhoto courtesyTMR Consulting

Intergranular Attack of Austenitic Stainless Steels:

With austenitic stainless steels, intergranular attack is usually the result of chromium carbide precipitation (Cr23C6) at grain boundaries, which produces a narrow zone of chromium depletion at the grain boundary. This condition is termed sensitization and it is shown schematically Figure 2.  Sensitization involves the precipitation of chromium carbides at grain boundaries, which results in a narrow zone of chromium depletion at the grain boundary.

Figure 2: Chromium depletion at the grain boundaries or sensitization

Because the chromium is the primary alloying element that makes stainless steel corrosion resistant, the chromium-depleted regions are susceptible to preferential corrosion attack. It is believed that this occurs because the chromium content immediately adjacent to the carbide may be below that required for the stainless steel alloy.  If the carbides form a continuous network on the grain boundary, then corrosion can produce a separation or gap at the boundary and possible grain dropping or loss.

Chromium Carbide Precipitation

The chromium carbides tend to precipitate at the grain boundaries of austenitic stainless steels in the 950 to 1450°F  temperature range.  Any exposure or thermal excursion into this temperature range during metal manufacture, fabrication, or service could potentially sensitize the steel.

Common practices such as welding, stress relief, and hot forming can expose the steel to the sensitizing temperature range.  The formation of chromium carbides is readily reversed by a solution anneal heat treatment. The test methods outlined in ASTM A262 have been developed to detect susceptibility to intergranular attack in austenitic stainless steels. 

The time and temperature required to produce susceptibility to intergranular attack (IGA) is dependent on alloy composition, particularly the carbon content.  Figure 3 shows the time-temperature-sensitization curves for Type 304 alloys with varying amounts of carbon content. 

Figure 3: Time-temperature-sensitization curves for Type 304 alloys as a function of carbon content
Image courtesy of the Nickel Institute

Three approaches have been used with the austenitic stainless steels to minimize to the effects of IGA.  Material that has been sensitized can be solution annealed by heating to a temperature where the carbides dissolved and the chromium-depleted regions are eliminated.   The carbon is then kept in solution by rapid cooling through the sensitizing temperature range. The recommended solution anneal temperature depends on the alloy and is typically done in the range of 1900 to 2150°F followed by rapid cooling.

Resistance to IGA can also be achieved by reducing the carbon content to below 0.030% level.  As shown in Figure 3, lower carbon contents move the nose of the time-temperature-sensitization curve to longer times.  The low carbon grades such as Types 304L, 316L, and 317L have been designed to resist sensitization during typical welding operations, but they do not resist sensitization by long term exposure in the critical temperature range in service.  The higher alloyed, more corrosion resistant stainless steels such as the 904L and 6Mo alloys have very low carbon contents and susceptibility to IGA is typically not a concern. 

The addition of stabilizing elements such as Ti, Nb (Cb), and Ta can also provide increased resistance to sensitization, especially for long-term exposures in the critical range in service.  These stabilizing elements tend to form carbides that are more stable than chromium carbide in the temperature range of 2250 to 1450°F.   So as the alloy cools from high temperatures, the carbon combines with the stabilizing elements and is unavailable for chromium carbide precipitation at the lower sensitizing temperature range of 950 to 1450 °F.  Common stabilized austenitic grades include Type 321, 347, 20-Cb3, and 316Ti.  Figure 4 summarizes the carbide precipitation reactions that occur in type 304 and 347 stainless steels. 

Figure 4: Precipitation Reactions in Type 304 and 347 Stainless Steel

Temperature Range	Precipitation Reactions
Melting point – 2250 °F	Niobium (Columbium) carbide dissolves Chromium carbides dissolves
2250 to 1450 °F	Columbium carbide precipitates Chromium carbides dissolves
1450 to 950 °F	Chromium carbides precipitates
950 to 70 °F	No reaction

With the stabilized grades, standard solution annealing treatments generally do not tie up all the available carbon.  So when the stabilized grades in the solution-annealed condition have long time exposures to the sensitizing temperature range (1450 to 950 °F), chromium carbide precipitation and sensitization can occur.  Stabilizing heat treatments can be used to more effectively tie up carbon by completing the precipitation reactions.  These treatments consist of holding the alloy for several hours in the 1500 to 1600°F temperature range.  [See ASTM A403, supplemental S10, p 301 volume 1.01.]

Intergranular Attack of Ferritic Stainless Steels:

Although intergranular attack of ferritic stainless steels is similar to that found in austenitic stainless steels, there are some important differences.  Because the solubility of nitrogen is low in the ferritic crystal structure, the precipitates that cause sensitization in ferritic grades include both chromium carbides (Cr23C6) and chromium nitrides (Cr2N). 

With the ferritic grades, sensitization occurs during cooling from higher temperatures (>1700 °F).  At these high temperatures the carbides and nitrides are put into solution and during cooling they can precipitate at grain boundaries resulting in chromium depletion.  The very high diffusion rates in the ferrite structure make it impossible to cool the steel fast enough to avoid precipitation of carbides and nitrides at grain boundaries.  For this reason, most commercial ferritic grades avoid sensitization by restricting the level of C and N and requiring the addition of stabilizing elements such as Ti, Ta, or Nb. 

If sensitization has occurred in a ferritic stainless steel, the condition can be “healed” by back diffusing chromium into the depleted regions.  “Healing” can be achieved by holding the material at 1100 – 1200 °F for several hours.  The test methods outlined in ASTM A763 have been developed to detect susceptibility to intergranular attack in ferritic stainless steels.

CORROSION: PITTING AND CREVICE CORROSION

Definitions:

Localized corrosion such as pitting and crevice corrosion of stainless steels generally occurs in the presence of halide ions, typically chloride (e.g. coastal and deicing chloride salts – sodium, calcium or magnesium chlorides; hydrochloric acid; bleach – sodium or calcium hypochlorite; and other chloride compounds). 

Pitting Corrosion

Pitting occurs when there is a localized breakdown of the stainless steel’s protective passive layer on an openly exposed surface.  Once initiated the growth rate of the pit can be relatively rapid resulting in deep cavities and even through-wall attack.  Other metals, such as aluminum, can also exhibit pitting corrosion.

Crevice Corrosion

Crevice corrosion occurs at locations where oxygen cannot freely circulate such as tight joints, under fastener heads and in other circumstances where the pieces of metal are in close contact.

Chloride salts, pollutants and moisture from the environment accumulate in the crevice. The environment inside the crevice becomes depleted of oxygen, enriched in chlorides, and acidified which promotes the breakdown of the passive film and anodic dissolution.

Environmental Factors:

The important environmental factors that favor localized attack are higher chloride content, higher temperatures, lower pH, and more noble corrosion potentials.

Figure 1: Effect of Temperature on the pitting resistance of S44660 super ferritic stainless steel in 6% ferric chloride test solution 
  Photograph courtesy of TMR Stainless

Figure 2:  Crevice corrosion inside a 304L stainless steel piping system which initiated in a crevice created by lack of a full penetration in an orbital weld.
Photograph courtesy of TMR Stainless

Figure 3: Crevice corrosion under a Type 316 bolt, which is holding a Type 316 structural beam in place, on the Florida coast

Photo courtesy of TMR Consulting

Influence of alloy Composition:

The resistance of a stainless steel to localized attack is strongly related to its alloy content.  The primary elements that contribute to the pitting and crevice corrosion resistance of austenitic and duplex stainless steels are chromium (Cr), molybdenum (Mo), tungsten (W), and nitrogen (N).  Tungsten, although not commonly used, is about half as effective on a weight percent basis as molybdenum in improving corrosion resistance. 

The Pitting Resistance Equivalent number or (PREn) has been developed to correlate a stainless steel’s composition to its relative pitting corrosion resistance.  The PRE relationship for austenitic and duplex stainless steels is usually given as follows.

          PRE = %Cr + 3.3%(Mo + 0.5W) + x%N
          where x is typically given as either 16 or 30

 
Although this relationship has been developed to rank pitting resistance, it also provides a relative ranking of a stainless steel’s crevice corrosion resistance. 

The PRE number is related to the grade’s resistance under ideal conditions and does not address factors such as the presence of intermetallic phases, improper heat treatments, or inferior surface condition.  Because of this limitation, one must be careful when using the PRE number to make material decisions.

The relative resistance to localized corrosion for different alloys can be quantified by determining the critical temperature required for initiating attack.  The critical pitting temperature (CPT) or critical crevice corrosion temperature (CCCT) is measured using either immersion test methods, such as those outlined in ASTM G48, or by electrochemical methods, such as the ASTM G150 test method.  When a higher critical temperature is necessary to initiate attack of a stainless steel alloy, it is more resistant to localized chloride attack.  These critical temperatures relate to standard laboratory environments and are not readily transferable to practical operating environments but they provide an indication of relative performance.

As is shown in Figure 4, the CCCT of any alloy is lower then the CPT. Figures 2 and 3 also illustrate that crevice corrosion can occur when the remaining fully exposed surface of the stainless steel is not exhibiting pitting corrosion. This makes designing to avoid or seal crevices an important factor in achieving the desired level of performance.

Figure 4: CPT and CCCT for various austenitic stainless steels, duplex stainless steel, and nickel alloys measured in 6% ferric chloride

Courtesy of the Nickel Institute

Avoidance

Pitting and crevice corrosion can be avoided by either choosing a more corrosion resistant stainless steel or by changing the service environment so that it is less aggressive.  For example, lower service temperatures or reduced levels of chloride can substantially reduce the aggressiveness of the environment.

Sealing or eliminating tight joints during design can often avoid crevice corrosion. In immersed applications or where standing water will be present, welding is the preferred solution. In true water shedding applications, like roofs and wall panels, flexible inert washers or construction sealants can often be used effectively but they should not be considered for water pooling or immersion. Figure 5 illustrates common tight crevice conditions and possible solutions.

Figure 5: Unsuitable metal design details for exterior atmospheric (not immersed) locations where chlorides will be present and typical solutions
Courtesy of the Nickel Institute

All metals can exhibit crevice corrosion but the service environment conditions under which it can occur and the location where metal loss occurs varies (e.g. inside or just outside the crevice) with the metal family. When two different metals create a tight crevice, both crevice and galvanic corrosion can potentially occur.  (See Galvanic Corrosion.)

CORROSION: GENERAL CORROSION

Definition:

The most common example of "general" or "uniform" corrosion is the attack that occurs when unprotected carbon steel is immersed in water or a mild acid. It is characterized by uniform thinning of the metal.

General corrosion of a stainless steel occurs when there is a wide spread breakdown of the protective passive film allowing the entire surface to experience active corrosion. It is rare to see general corrosion of stainless steel.

Because of the relatively high corrosion resistance of stainless steels, it typically requires a very aggressive environment such as strong acids or bases to produce this mode of attack. The uniform corrosion rate usually increases with higher temperatures and increased flow rates.

The most common cause of general corrosion on stainless steel is the use of an inappropriate alloy with insufficient resistance for the environment.

CORROSION: GALVANIC CORROSION

Definition:

When two different metals or alloys are immersed in a corrosive solution or regularly connected by moisture, each will develop a corrosion potential. If the conditions for galvanic corrosion are present, the more noble metal will become the cathode and the more active metal will become the anode. A measurable current may flow between the anode and the cathode. If this occurs, the anode's rate of corrosion in the service environment will be increased while the cathode's corrosion rate will decrease. The increased corrosion of the anode is called "galvanic corrosion".

Galvanic corrosion is sometimes used to extend the life of materials (i.e. zinc coatings on carbon steel and zinc anodes in water heaters), but, if it is not considered and the right conditions exist, it can lead to unexpected failures.

Requirements for Galvanic Corrosion:

In order for galvanic corrosion to occur, three elements are required.

1. Two metals with different corrosion potentials
2. Direct metal-to-metal electrical contact
3. A conductive electrolyte solution (e.g. water) must connect the two metals on a regular basis. The electrolyte solution creates a "conductive path". This could occur when there is regular immersion, condensation, rain, fog exposure or other sources of moisture that dampen and connect the two metals.

If any of these elements is missing, galvanic corrosion cannot occur. If, for example, the direct contact between the two metals is prevented (plastic washer, paint film etc.) or if there is some other interruption in the conductive path, there cannot be galvanic corrosion and each metal will corrode at its normal rate in that service environment. Figure 1 shows examples of conditions that do not meet the all requirements for galvanic corrosion.

Figure 1: Examples of bi-metalic combinations when galvanic corrosion cannot occur

When two different metals are coupled together in atmosphere or water, the likelihood of developing galvanic corrosion can be predicted using a "galvanic series". In specialized applications, such as when dissimilar metals are embedded in concrete, corrosion data for that specific environment should be used.

Figure 2 shows the galvanic series measured in seawater for some common metals and alloys. When two metals are further apart in the list (e.g. a larger difference between the two numbers), the driving force for galvanic corrosion is increased. The most anodic (active) metals are at the top and most cathodic (noble) at the bottom. Both solid and hollow bars are shown for the stainless steels. The hollow bars represent actively corroding stainless steel, which has a different potential then passive (not corroding) stainless steel. In most applications, where dissimilar metals are combined, the passive (solid) bar should be used to determine the position of the stainless steel.

For example, if zinc (think galvanized steel) which is an active material and near the top of the list and stainless steel, a noble metal and near the bottom of the list were in direct contact and in the presence of an electrolyte (water), galvanic corrosion will occur if they are regularly exposed to an electrolyte.

Figure 2: Galvanic Series in Seawater (insert galvanic series image.gif)

Relative Surface Area

In addition to the three elements sighted above, the relative surface area (not mass) of each of the exposed metals is also an important factor. (See Figure 3). If the area of the cathode (noble metal) is very large, and the anode (active metal) is very small, the current produced is likely to be very high and the anode will corrode quickly.

For example, if a window frame made of stainless steel and it is attached with carbon steel screws, the screws will probably corrode at an accelerated rate. If the area of the cathode (noble metal – stainless steel) is very small, and the anode (active metal – carbon steel) is very large, the current produced will be very low and the corrosion rate of the anode may not be affected. If the window frame is made of carbon steel and it is attached with stainless steel screws there will be very little, if any, galvanic corrosion.

Figure 3A shows the galvanic corrosion of carbon steel bolts used to secure a stainless steel structural railing support on a bridge. The small surface area of the active bolts results in an undesirable galvanic couple and they are exhibiting an accelerated corrosion rate. Image 3B shows stainless steel fasteners used to secure a carbon steel tread plate. The relatively small surface area of the stainless steel fasteners means that they have essentially no galvanic effect on the corrosion rate of the carbon steel plate.

Figure 3: Examples of good and bad galvanic corrosion ratios

A: photograph courtesy of GKD, B: photograph courtesy of TMR Consulting)

Dissimilar metal combinations should be avoided in areas where moisture is likely to accumulate and remain for long periods. In well-drained exterior applications, dissimilar metals can be used together if favorable surface ratios exist, but the best solution is to electrically insulate one from the other. When painted carbon steel and stainless steel are welded together, the welded joint should be painted. Stainless steel fasteners with neoprene or other inert washers are regularly used with other metals.

Metal Combination Examples

The Statue of Liberty (completed in 1886) is one of the highest profile examples of the damage that galvanic corrosion can cause. The original design used a copper exterior skin (large cathodic or noble surface area) supported by a cast iron structural frame (small anodic or active surface area) with the metals separated by wool felt which eventually failed. In 1984, it was closed to the public due to significant corrosion of the cast iron frame. It was rebuilt using a duplex stainless steel structural frame. Copper alloys and stainless steels are quite close in the galvanic series with the duplex being more cathodic which is appropriate since it has the smaller surface area ratio.

Aluminum adjoins zinc in the galvanic series. Decision makers are often aware that aluminum's corrosion rate in atmosphere is between that of carbon steel and stainless steel, but, when it is directly coupled to another metal and an electrolyte is present on a regular basis, it becomes very anodic (active) and it will corrode at a higher rate than either carbon steel or stainless steel which are both more cathodic (noble). For that reason, if aluminum structural framing is used to support sheets of stainless steel and they are in direct contact with moisture present, the aluminum's corrosion rate can be accelerated.

When multiple metals must be combined in direct contact and an electrolyte is likely to be present, the fasteners should always be specified to match the most noble of the metals being joined.

Not all environments produce the same galvanic results. Research has shown that galvanic corrosion is not a concern between stainless and carbon steel in concrete.

CORROSION: Chloride Stress Corrosion Cracking

Definition:

The combination of tensile stress and a specific corrosive environment can crack stainless steels.  This mode of attack is termed stress corrosion cracking (SCC). The most common environmental exposure condition responsible for SCC of stainless steels is the presence of chlorides.  Although no stainless steel grade is totally immune to chloride SCC, the relative resistance of stainless steels varies substantially.

Influence of Alloy Composition:

The relative resistance to chloride SCC is dependant on the stainless steel family.  The austenitic family of stainless steels is the most susceptible.  The resistance of austenitic stainless steels to SCC is related to the nickel content of the steel. 

The most susceptible austenitic grades have nickel contents in the range of 8 to 10 wt%.  Therefore, standard grades such as 304/304L and 316/316L are very susceptible to this mode of attack.  Austenitic grades with relatively high nickel and molybdenum contents such as alloy 20, 904L, and the 6% molybdenum super austenitic grades have substantially better chloride SCC resistance.

The ferritic family of stainless steels, which includes grades such as type 430 and 444 are very resistant to chloride SCC.  The duplex stainless steel with their dual austenite/ferrite microstructures has a resistance that is in between that of the austenite and ferrite grades.

Corrosion Testing

The relative resistance of a stainless steel to chloride SCC is often quantified by the use of standard boiling salt solutions.  The following table summarizes the results of testing in boiling salt solutions of 26% NaCl (sodium chloride), 33% LiCl (lithium chloride), and 42% MgCl2 (magnesium chloride).  The boiling LiCl and MgCl2 test solutions are very aggressive relative to practical applications and only austenitic alloys with compositions that approach those of nickel-base alloys will routinely resist cracking in these test solutions.

Table 1: Relative chloride SCC resistance measured using fully immersed U-bend specimens in standard boiling salt solutions. (Taken from producer data)

Alloy	42% MgCl2	33% LiCl	26% NaCl
Austenitic SST
Type 304L (S30403)	SCC	SCC	SCC
Type 316L (S31603)	SCC	SCC	SCC
904L (N08904)	SCC	SCC	No Cracking
6% Mo SST	SCC	SCC	No Cracking
Alloy 20 (N08020)	SCC	No Cracking	No Cracking
Duplex SST
2205 (S32205)	SCC	No Cracking	No Cracking
255 (S32550)	SCC	---	No Cracking
2507 (32750)	SCC	---	No Cracking
Ferritic SST
439 (S43035)	No Cracking	No Cracking	No Cracking
444 (S44400)	No Cracking	No Cracking	No Cracking

Crack Appearance

The typical crack morphology for chloride stress corrosion cracking consists of branched transgranular cracks.  Figure 1 shows the cracking that occurred on a 6Mo super austenitic stainless steel  (N08367) exposed to 0.2% chlorides at 500 °F (260 °C)

Figure 1: Typical appearance of chloride stress corrosion cracking

Photo courtesy of TMR Stainless

Environmental Factors:

The environmental factors that increase the cracking susceptibility include higher temperatures, increased chloride content, lower pH, and higher levels of tensile stress.  Temperature is an important variable.  When stainless steels are fully immersed, it is rare to see chloride stress corrosion cracking at temperatures below 60 °C (150 °F). 

There is a synergistic relationship between dissolved oxygen and the chloride level.  If the oxygen level is reduced to the 0.01 to 0.1 ppm range, aqueous solutions containing low to moderate chloride levels are not likely to crack austenitic alloys, such as 304L and 316L.  The normal solubility of O2 in water at room to moderate temperatures (e.g. up to 140°F/60°C) is 4.5 to 8 ppm at atmospheric pressure.

In actual service environments, evaporation can produce local build-up of aggressive corrosive substances, such as chlorides and the H+ ions, resulting in conditions that are substantially more aggressive.  Under severe evaporative conditions, stainless steels can crack at temperatures well below the thresholds measured under conditions where there is full immersion.  Because of this, one must use caution when specifying materials for applications that involve the evaporation of chloride-bearing solutions on hot stainless steel surfaces.

The Materials Technology Institute (MTI) of the Chemical Process Industry has reviewed literature and collected case histories to define guidelines for the chloride SCC susceptibility of types 304L and 316L stainless steel in neutral water environments. 

Figure 2 shows the cracking threshold for 304L and 316L stainless steel as a function of temperature and chloride content.  The level of chlorides required to produce cracking is relatively low.  Failures have been reported in environments with as little as 10 ppm chlorides.  This is particularly true for environments having concentrating (evaporating) mechanisms such as wet/dry interfaces or a film of solution in immediate contact with a heat-rejecting surface.  In these situations, a few ppm of chlorides in the bulk solution can concentrate to hundreds of ppm in the area of evaporation.

Figure 2: Cracking threshold for 304 and 316 alloys exposed to near neutral chloride-bearing waters

The cracking threshold of a 6Mo super austenitic stainless steel (UNS N08367) immersed in oxygen-bearing neutral chloride solutions is shown in Figure 3.  The temperature thresholds are well above the 212°F (100°C) range, indicating that exposures to atmospheric boiling in neutral chloride solutions are very unlikely to produce cracking.

Figure 3: Cracking threshold for a 6Mo super austenitic steel ( UNS N08367) immersed in neutral NaCl solutions.

Courtesy of TMR Stainless

Definition:

The combination of tensile stress and a specific corrosive environment can crack stainless steels.  This mode of attack is termed stress corrosion cracking (SCC). The most common environmental exposure condition responsible for SCC of stainless steels is the presence of chlorides.  Although no stainless steel grade is totally immune to chloride SCC, the relative resistance of stainless steels varies substantially.

Influence of Alloy Composition:

The relative resistance to chloride SCC is dependant on the stainless steel family.  The austenitic family of stainless steels is the most susceptible.  The resistance of austenitic stainless steels to SCC is related to the nickel content of the steel. 

The most susceptible austenitic grades have nickel contents in the range of 8 to 10 wt%.  Therefore, standard grades such as 304/304L and 316/316L are very susceptible to this mode of attack.  Austenitic grades with relatively high nickel and molybdenum contents such as alloy 20, 904L, and the 6% molybdenum super austenitic grades have substantially better chloride SCC resistance.

The ferritic family of stainless steels, which includes grades such as type 430 and 444 are very resistant to chloride SCC.  The duplex stainless steel with their dual austenite/ferrite microstructures has a resistance that is in between that of the austenite and ferrite grades.

Corrosion Testing

The relative resistance of a stainless steel to chloride SCC is often quantified by the use of standard boiling salt solutions.  The following table summarizes the results of testing in boiling salt solutions of 26% NaCl (sodium chloride), 33% LiCl (lithium chloride), and 42% MgCl2 (magnesium chloride).  The boiling LiCl and MgCl2 test solutions are very aggressive relative to practical applications and only austenitic alloys with compositions that approach those of nickel-base alloys will routinely resist cracking in these test solutions.

Table 1: Relative chloride SCC resistance measured using fully immersed U-bend specimens in standard boiling salt solutions. (Taken from producer data)

Alloy	42% MgCl2	33% LiCl	26% NaCl
Austenitic SST
Type 304L (S30403)	SCC	SCC	SCC
Type 316L (S31603)	SCC	SCC	SCC
904L (N08904)	SCC	SCC	No Cracking
6% Mo SST	SCC	SCC	No Cracking
Alloy 20 (N08020)	SCC	No Cracking	No Cracking
Duplex SST
2205 (S32205)	SCC	No Cracking	No Cracking
255 (S32550)	SCC	---	No Cracking
2507 (32750)	SCC	---	No Cracking
Ferritic SST
439 (S43035)	No Cracking	No Cracking	No Cracking
444 (S44400)	No Cracking	No Cracking	No Cracking

Crack Appearance

The typical crack morphology for chloride stress corrosion cracking consists of branched transgranular cracks.  Figure 1 shows the cracking that occurred on a 6Mo super austenitic stainless steel  (N08367) exposed to 0.2% chlorides at 500 °F (260 °C)

Figure 1: Typical appearance of chloride stress corrosion cracking

Photo courtesy of TMR Stainless

Environmental Factors:

The environmental factors that increase the cracking susceptibility include higher temperatures, increased chloride content, lower pH, and higher levels of tensile stress.  Temperature is an important variable.  When stainless steels are fully immersed, it is rare to see chloride stress corrosion cracking at temperatures below 60 °C (150 °F). 

There is a synergistic relationship between dissolved oxygen and the chloride level.  If the oxygen level is reduced to the 0.01 to 0.1 ppm range, aqueous solutions containing low to moderate chloride levels are not likely to crack austenitic alloys, such as 304L and 316L.  The normal solubility of O2 in water at room to moderate temperatures (e.g. up to 140°F/60°C) is 4.5 to 8 ppm at atmospheric pressure.

In actual service environments, evaporation can produce local build-up of aggressive corrosive substances, such as chlorides and the H+ ions, resulting in conditions that are substantially more aggressive.  Under severe evaporative conditions, stainless steels can crack at temperatures well below the thresholds measured under conditions where there is full immersion.  Because of this, one must use caution when specifying materials for applications that involve the evaporation of chloride-bearing solutions on hot stainless steel surfaces.

The Materials Technology Institute (MTI) of the Chemical Process Industry has reviewed literature and collected case histories to define guidelines for the chloride SCC susceptibility of types 304L and 316L stainless steel in neutral water environments. 

Figure 2 shows the cracking threshold for 304L and 316L stainless steel as a function of temperature and chloride content.  The level of chlorides required to produce cracking is relatively low.  Failures have been reported in environments with as little as 10 ppm chlorides.  This is particularly true for environments having concentrating (evaporating) mechanisms such as wet/dry interfaces or a film of solution in immediate contact with a heat-rejecting surface.  In these situations, a few ppm of chlorides in the bulk solution can concentrate to hundreds of ppm in the area of evaporation.

Figure 2: Cracking threshold for 304 and 316 alloys exposed to near neutral chloride-bearing waters

The cracking threshold of a 6Mo super austenitic stainless steel (UNS N08367) immersed in oxygen-bearing neutral chloride solutions is shown in Figure 3.  The temperature thresholds are well above the 212°F (100°C) range, indicating that exposures to atmospheric boiling in neutral chloride solutions are very unlikely to produce cracking.

Figure 3: Cracking threshold for a 6Mo super austenitic steel ( UNS N08367) immersed in neutral NaCl solutions.

Courtesy of TMR Stainless

Local Weather Report and Forecast For: Kakinada Dated :Aug 28, 2019

KAKINADA

EXTREME WEATHER EVENTS IN THE MONTH OF AUGUST

Year	Temperature(oC)		Rainfall (mm)
	Highest Maximum(Date)	Lowest Minimum(Date)	24 Hours Highest (Date)	Monthly Total
2018	36.3(5)	23.4(9)	29.4(14)	125.7
2017	36.2(2)	24.2(30)	26.9(3)	105.8
2016	36.6(21)	24.2(24)	39.2(24)	53.2
2015	36(25)	24.3(4)	65.6(17)	184.1
2014	36.7(9)	24.5(13)	37(29)	101.1
2013	36(31)	21.7(1)	22(14)	69.4
2012	35.2(17,18)	23.6(4)	55.4(30)	278.8
2011	35.4(13)	23.6(3)	68(3)	232.2
2010	36.4(11)	23(31)	49(13)	241.1
2009	38.4(4, 7)	24.2(20)	44(31)	141.8
ALL TIME RECORD	38.4(4,7/2009)	21.7(19/1955 & 01/2013)	155.8(08/1983)	374.8(1983)

CLIMATOLOGICAL TABLE

PERIOD: 1981-2010

Month	Mean Temperature(oC)		Mean Total Rainfall (mm)	Mean Number of Rainy Days	Mean Number of days with
	Daily Minimum	Daily Maximum			HAIL	Thunder	FOG	SQUALL
Jan	20.3	29.2	12.6	0.9	0	0.1	0.1	0
Feb	21.7	31.2	10.3	1.1	0	0	0	0
Mar	24.0	34.0	7.5	0.5	0	0.7	0.1	0
Apr	26.2	36.2	16.4	1.1	0	2.1	0	0
May	27.8	37.5	42.3	2.8	0	5.2	0	0
Jun	27.3	35.8	122.8	7.1	0	5.4	0	0
Jul	26.2	32.9	175.4	10.9	0	5.2	0	0
Aug	25.9	32.2	176.9	10.2	0	4.7	0	0
Sep	25.9	32.7	199.4	9.1	0	7.5	0	0
Oct	24.8	31.9	243.4	9.0	0	6.8	0	0
Nov	22.5	30.4	98.8	3.8	0	1.3	0	0
Dec	20.3	29.2	10.7	0.9	0	0	0	0
Annual	24.4	32.8	1116.6	57.5	0	39	0.1	0

Local Weather Report and Forecast For: Kakinada    Dated :Aug 28, 2019

Past 24 Hours Weather Data Maximum Temp(oC) (Recorded. on 28/08/19) 34.3 Departure from Normal(oC) 1 Minimum Temp (oC) (Recorded. on 28/08/19) 27.5 Departure from Normal(oC) 1 24 Hours Rainfall (mm) (Recorded from 0830 hrs IST of yesterday to 0830 hrs IST of today) NIL Relative Humidity at 0830 hrs (%) 83 Relative Humidity at 1730 hrs (%) (Recorded. on 28/08/19) 68 Todays Sunset (IST) 18:18 Tommorows Sunrise (IST) 05:47 Moonset (IST) 16:40 Moonrise (IST) 03:15

7 Day's Forecast Date Min Temp Max Temp Weather 28-Aug 27.0 34.0 Generally cloudy sky with Light rain 29-Aug 27.0 34.0 Generally cloudy sky with Light rain 30-Aug 26.0 33.0 Generally cloudy sky with moderate rain 31-Aug 26.0 32.0 Generally cloudy sky with one or two spells of rain or thundershowers 01-Sep 26.0 32.0 Generally cloudy sky with Heavy rain 02-Sep 27.0 33.0 Rain 03-Sep 27.0 33.0 Rain