There are thousands of sources of literature on this question. But in a few words, it can be summarized as follows:

1. Steel corrodes almost always only by pitting. It's always an electrochemical (galvanic) corrosion and not erosion. This corrosion proceeds in electrolytes - aqueous solutions of various acids and salts.

2. Cause corrosion of stainless steels is the local losing of passivation an the surface (Thin Cr2O3 layer, that normally protects the surface, is destroyed in some places by means of corrosive medium).

3. Mechanism of corrosion is to form a galvanic element between passivated (cathode) and not passivated (anode) surfaces.

Thank you so much Sir. For this question, the medium is condensate water (Blow down). this fluid is truly not corrosive. C-scan mapping was performed and the results show big reduction of wall loss. so now, I want to find the main root cause of the thinning problem.

What is the impingement velocity of the blow-down?  You could be experiencing cavitation or erosion corrosion damage -  even in truly benign/non-oxidizing media, 304 and 316 can suffer material loss via pure cavitation - research has shown that the formation and collapse of micro-bubbles produce shock-waves that are stronger than the tensile strength of a passivated SS.  Im not sure of your equipment's configuration, but one option is to install a sacrificial impingement plate of similar material of construction (MOC) or to use an applied, erosion resistant coating (typically flame sprayed - see "HVOF" coatings.  If you have access to perform visual inspection of the damaged surface, you should be able to categorize the damage as cavitation, which has a very distinctive appearance - usually starting off with a 'frosted' appearance of the surface, and later developing into a crater-like morphology.  - Good luck!

Thank you so much Sir. FYI, the equipment for 304 and 316 are used for blowdown tank and blowdown cooler (Heat Exchanger)  respectively. At this moment we cant access the equipment yet and need to get the real root cause why thinning occurs. Based on API 571, Cavitation and Erosion-Corrosion might be the factor of thinning. However, We dont know the range of erosion velocity that can cause of material damage. Sir, what is the acceptable range of velocity for 304 and 316? 

I agree with Vadim's answer.