# Inspection of Coated Hydrogen Transportation Pipelines
## 4 Evaluation of Current Pipeline Inspection Methods for Hydrogen Pipelines
### Coatings
#### 4.2.2 Monitoring Coating Health  
There is a need to ensure that coatings are operational and effective. This means that monitoring coating health becomes necessary. In this effort, considering standards allows one to plan how this might be done. ISO 9587:2007, ISO 9588:2007, GB/T 13322-1991, GB/T 19349-2012 and GB/T 19350-2012 are possible standards that could be used to ensure the protection against HE provided by the coatings is satisfactory [105].  
Cumulatively, considering the standards for both HE protection and testing, there are a number of tests to be considered, some of which include [105]:  
* The \"Copper-Hydrogen embrittlement test\"
* The \"inclined wedge method\" to test for residual embrittlement
* The \"incremental step loading method\" to test for hydrogen embrittlement threshold  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Type of Coating** & **Description** \\ \hline Metallic coatings [102; 103] & Electroplating high strength steels with cadmium (Cd) or zinc (Zn) can act to mitigate corrosion [102]. However, this process can cause hydrogen to enter the steel; hence, the specimen needs to be put through de-embritting’ by heating for about 8 to 24 h to enable “diffusible” hydrogen to leave the metal [102]. In the case of 304 austenitic stainless steel, aluminium (Al), copper (Cu), nickel (Ni) and Zn were electroplated onto the specimen with differing degrees of success [103]. Zn and Ni, for instance, were found to be ineffective at protecting the underlying steel against HE (Hydrogen Environment embrittlement) [103]. These coatings were found to have fractured at “very low strains” in tensile testing, causing the specimen to come into into contact with hydrogen, resulting in the initiation of HEE [103]. Cu appeared to be the more successful coating— with commendable tensile ductility, and adhesion [103]. However, parts with improper coating and “pinholes” were found to act as points of failure for the coating [103]. \\ \hline Non-metallic coatings [102; 103; 104] & Coatings made of compounds were found to have significant utility. Black oxide is shown to lower the ingress of atomic hydrogen into steel and slow down the formation of surface cracks arising from lubricants [102]. Aluminium consisting of MAX phase coatings was also found to be functional as a “protective coating” at high temperatures [104]. NiP and Ti-DLC coatings were found to have the same problems seen in “on top” coatings of metals: cracking was seen at low strains, and hydrogen could come into contact with the specimen being protected [103]. Categorised as “hard coatings”, compounds such as TiC, TiN, BN, TiO\({}_{2}\) and WC are able to act as effective barriers against hydrogen ingress [102]. However, their effectiveness is dependent on “service conditions”, the presence of defects in the coatings and structural integrity of the coatings [102]. \\ \hline \hline \end{tabular}
\end{table}
Table 9: Categorisation of coatings and their effectiveness.  
However, given that this review focuses on NDT, the possibility of inspecting coatings using NDT is also considered in the following sub-section.