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High-temperature hot corrosion has been known since the 1950s. It is an extremely rapid form of oxidation that takes place at temperatures between 1500 oF/816 oC and 1700 oF/927 oC in the presence of sodium sulfate (Na2SO4). Sodium sulfate is generated in the combustion process as a resultof the reaction between sodium， sulfur， and oxygen. Sulfur is present as a natural contaminant in the fuel.
Table 11-1 Operation and Maintenance Life of an 1ndustrial Turbine

Firing Temperature below  Firing Temperature above 
Type of .pplication  1700 0F (927 0C)  170. 0F (92. 0C. 
and Fuel  Comb.  1st Stage  1st Stage  Comb.  1st Stage  1st Stage 
Liners  Nozzle  Blades  Liners  Nozzle  Blades 
BASE LOAD  Starts/hr  +  +  + 
Nat. gas  1/1000 30，00 0 60，00 0 100，0 00 15，00 0 25，00 0 35，00 0 
Nat. gas  1/10  7500 42，00 0 72，000 3，750 20，00 0 25，00 0 
Distillate oil  1/1000 22，00 0 45，00 0 72，000 11，25 0 22，00 0 30，00 0 
Distillate oil  1/10  6000 35，00 0 48，000 3，000 13，50 0 18，00 0 
Residual  1/1000 3，500 20，00 0 28，000 2，500 10，00 0 15，00 0 
Residual  1/10 
SYSTEM PEAKING 
Normal max. load of 
short duration and daily starts 
Nat. gas  1/10 7，500 34，00 0 60，000 5，000 15，00 0 24，00 0 
Nat. gas  1/5 3，800 28，00 0 40，000 3，000 12，50 0 18，00 0 
Distillate  1/10 6，000 27，20 0 53，500 4，000 12，50 0 19，00 0 
Distillate  1/5 3，000 22，40 0 32，000 2，500 10，00 0 16，00 0 
TURBINE PEAKING 
Operating Above 
50 oF-100 oF (28 oC-56 oC) 
Firing Temperature 
Nat. gas  1/5 2，000 12，00 0 20，000 2，000 12，50 0 18，00 0 
Nat. gas  1/1  400 9，000 15，000  400 10，00 0 15，00 0 
Distillate  1/5 1，600 10，00 0 16，000 1，700 11，00 0 15，00 0 
Distillate  1/1  400 7，300 12，000  400 8，500 12，00 0 

Materials 41.
Low-temperature hot corrosion was recognized as a separate mechanism of corrosion attack in the mid-1970s. This attack can be very aggressive if the conditions are right. It takes place at temperatures in the 1100 oF (593 oC) to 1400 oF/760 oC range and requires a significant partial pressure of SO2.Itis caused by low melting eutectic compounds resulting from the combination of sodium sulfate and some of the alloy constituents such as nickel andcobalt. Itis， in fact， somewhat analogous to the type of corrosion called Fireside Corrosion in coal-fired boilers.
The two types of hot corrosion cause different types of attack. High-temperature corrosion features intergranularattack， sulfide particles and a denuded zone of base metal. Metal oxidation occurs when oxygen atoms combine with metal atoms to form oxide scales. The higher thetemperature， the more rapidly this process takesplace， creating the potential for failure of the component if too much of the substrate material is con-sumed in the formation of these oxides.
Low-temperature corrosion characteristically shows no denuded zone， nointergranular attack， and a layered type of corrosion scale.
The lines of defense against both types of corrosion are similar.First， reduce the contaminants.
Second， use materials that are as corrosion-resistant as possible.Third， apply coatings to improve the corrosion resistance of the bucket alloy.
Hot corrosion includes two mechanisms:
1.  Accelerated Oxidation
During initial stages-blade surface clean
Na2SO4 + Ni (metal) → NiO (porous)

2.  Catastrophic Oxidation
Occurs withMo，認(rèn)， and V present-reduces
NiO layer-increases oxidation rate

　
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