Nitric acid damage due to Partial Discharge

During PD activity in air, Ozone (O3) and Nitrogen Oxides (NOX) are generated, particularly Nitric Oxide (NO) on the surface of organic insulators. When exposed to moisture, such as in high-humidity environments, Nitric Oxide reacts to form highly corrosive nitric acid (HNO3). This nitric acid, when present on the insulator surface, initiates electrolytic erosion and accelerates the aging process of the polymer surface by disrupting molecular bonds or causing chain scission. Consequently, this leads to surface roughening and eventual loss of hydrophobicity, resulting in the surface becoming hydrophilic over time.?[1]

This leads to increased moisture and conductivity on the insulator surface, which in turn increases leakage current, partial arcs, and dry bands. These effects contribute to surface tracking that will eventually cause flashover and an HV Fault.??

Video shows nitric acid damage on 33kV termination

Metal components are prone to corrosion when exposed to nitric acid. Consequently, conductors and other metallic elements commonly exhibit noticeable signs of extensive rust and oxidation in proximity to partial discharge (PD) sites.?

Damage inflicted by nitric acid on insulator surfaces frequently manifests as a white powder-like substance. In severe cases, this damage cannot be easily wiped away. Even if the damage is relatively minor, superficial cleaning of the insulator may be possible, but the underlying harm persists, leading to ongoing degradation.?

More photos showing HV assets damaged by nitric acid:

Author: Dr Colin Smith, Managing Director, IPEC

[1] Elizabeth Da Silva Domingues “Improved Condition Monitoring of Composite Insulators” Ph.D. Thesis, School of Electrical and Electronic Engineering, The University of Manchester, Manchester, UK, 2012.