Top sealant coat - Usefulness

[Chakraborty_P]

Dear Gordon,

I have joined this forum recently & has gone thru the archived posts. I found it very useful and informative.

What I have understood is that for neutral (Ceramic etc.) and Anodic coatings (Al / Al+Mg / Zn), Top Sealant coating may not be necessary. However for Cathodic coating, a Top sealant coat is very much necessary.

To combat dew point corrosion in a Refinery column (DHDT Stripper Column - service - diesel+wild naphtha+ very high H2S + Ammonium bisulphide -NH4HS+ some possible chloride and water), we had planned to carry out metal spray with SS 316 - 3mm thick coating + Top sealant coat. The bond coat on substrate - Inconel/ 309-Mo.

As I understand, a 3mm thick SS316 coating will have shrinkage defects and porosity. Hence, top coating is a must. Probably, I should have kept the thickness of coating limited to 1.5 mm.

In case I decide to do away with top coating, I have to move Anodic coating only. The forum members have advised me to use Al-coating (TSA) with ARC spary with max 300 micron thickness & top sealant coat. In case of TSA, may questions are -

1) Whether I can do away with top sealant coat ?
2) Whether a bond coat (between substrate & Al-coat) will be required ? If so, what type & thickness ?
3) What is the thickness/coat for layers I adhere to ?

With best regards


Chakraborty

[Gordon]

Hi Chakraborty

What I have understood is that for neutral (Ceramic etc.) and Anodic coatings (Al / Al+Mg / Zn), Top Sealant coating may not be necessary.

For ceramic coatings I would recommend sealing. Neutral coatings do not provide sacrificial protection. Anodic coatings are not totally reliant on excluding the environment for providing corrosion protection as with other coatings, but sealing them can/may improve coating life.

metal spray with SS 316 - 3mm thick coating

I have my doubts that you would ever be able to achieve such a thickness with 316 without it failing. Even 1.5mm thickness would be risky.

1) Whether I can do away with top sealant coat ?

Possibly, but it may reduce overall coating life.

2) Whether a bond coat (between substrate & Al-coat) will be required ? If so, what type & thickness ?

Normally no

3) What is the thickness/coat for layers I adhere to ?

About 25 micrometres per pass

[Chakraborty_P]

Dear Gordon,

Thanks a lot. The replies really helped me to conclude.

regards

Chakraborty