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Is there any NDT technique acceptable worldwide as per codes & standards which can be used to check Hydrogen attack in low alloy steels? Any specific temperature & pressure conditions at which these steels are prone to this attack?

You need special NDT techniques in order to detect this highly localized type of attack.
To start with UT is the best suitable NDT technique as you have to find out fissuring in the base metal and UT is best in detecting crack growth. High frequency shear wave angle beam spectrum analysis UT techniques shall be used if the attack is in the initial stage.
In the advanced stages of hydrogen attack, conventional shear wave UT and TOFD can be used.
When the internal surface is accessible, you can go for wet fluorescent magnetic particle testing and in-situ metallography. For details, refer to Art. 6 of API RP 941. Everything has been listed in detail. It tells you about the inspection frequency also for specific low alloy steels used in hydrogen service.

Yes the combination of hydrogen partial pressure and the temperature determines the chance of hydrogen attack for a specific steel. Refer to Fig. 1 in API RP 941. This API is the key document about the information you need. For your steel and operating limits, you can find everything in the same.

Hope this would help.