Corrosion resistance overlay of Inconel 625 on SS 316L base materials for Sea Water Application? Any pitfalls in the process?
Inconel 625, a Ni-based solid solution strengthened alloy well known for its excellent strength and good corrosion resistance at extreme environments used in thermal plants, boiler tubes, petrochemical industry and power plant.
Nickel Alloy 625 is used both for its high strength and outstanding aqueous corrosion resistance, having outstanding resistance to chloride pitting and crevice corrosion cracking and immunity to chloride ion stress corrosion cracking (Ch-SCC)
The presence of Cr content (approximately 20 wt %) along with Mo-rich, Nb and Fe makes this Inconel 625 (Ni–Cr–Mo–Nb austenitic alloy achieve excellent corrosion resistance property.
SS 316 L is low carbon version of 316, a standard Mo-containing grade, next to importance of SS 304 amongst the austenitic stainless steels, most immune from sensitization (grain boundary carbide precipitation).
In SS 316 L, the Mo gives 316 better overall corrosion resistant properties than grade 304, particularly higher resistance to pitting and crevice corrosion in chloride containing environments. So, the selection of the base metal SS 316L and Inconel 625 filler metal is most appropriate with respect to the service application in sea water but here, the prime concern is about maintaining the Inconel 625 chemistry on the top overlay surface , i.e. functional surface.
During the overlay welding process a dilution of the Weld overlay filler metal with the base metal will take place. Generally, in order to save the cost of precious filler metal like Nickel based alloys, the fabricators would try to keep the overlay layer as thin as possible. While doing so, the fabricator tends to apply higher Heat input to complete the process in a single pass, which may result in a higher dilution of contents from base material in the chemistry of the deposited weld metal. As a result, changed chemical composition of the overlay material will also change the corrosion resistance. Dilution affects the PREN number, which is an index of the relative corrosion resistance of the material as calculated by the following formula:
PREN = %Cr + 3.3%Mo + 16%N
So, the purpose of overlay is not fulfilled for the given application.
Many literatures (Ref.1-6) do suggest that process of Inconel 625 overlay on CS, LAS and austenitic SS using conventional Arc welding processes, like GTAW (Hot/Cold Wire) , GMAW, FCAW and SAW including, Electro-strip cladding process are resulting in higher dilution in the first two layers so their corrosion performance is often compromised with “more than 5% Fe content “ diluted from the base metal into the Inconel 625 weld overlay chemistry.
The reason why Fe content badly affects the corrosion resistance of Ni based alloy is rather complex and requires extensive research study. Probably, one of the reasons is that Fe increases susceptibility to Chloride ion attack on corrosion resistant overlay of alloy 625, so as a result, the fabricated product suffers from issues like severe pitting and stress corrosion cracking.
-In order to keep the ion (Fe) content within the specified limit (less than or equal to 5% as reported by many literatures) there are some useful guidelines obtained through various researchoutcomes as under:-
-At least, two layers deposition (or 3 layers, until you get the pure Inconel 625 chemistry on the top layer) with controlled heat input or low heat input arc welding process, for example one of the processes like Cold Metal Transfer(CMT), patented by Fronius International) offering a droplet deposition mechanism, which controls the short circuit by the retraction motion of the wire from short circuiting action. There are electronically (digital wave form technology enabled) controlled short- circuit MIG welding process variants as well as high-deposition TIG welding variants giving the benefits of high-deposition rate and good control on dilution, reduced heat input, stable arc, uniform weld profile and a spatter free welding.
-The study conducted by Evangeline A et al (Ref.3) on Inconel 625 overlay on SS 316L using CMT reported successful weld overlay with optimum corrosion properties : Current=180 Amps, Voltage=17 volts, Torch Angle=80° and Welding speed=150 mm/min, attains less dilution (9.3%) and more clad bead width (8.956mm).
-Use of Solid (GMAW preferably) Inconel 625 weld consumables, preferably, already low in Fe content in undiluted weld metal chemistry. You, your client and your filler metal supplier have to sit together to figure out the permissible limit of Fe% in top undiluted layer being tolerated without affecting the performance of the weld overlay components. Probably, you can sit with filler wire manufacturer- technical representative to work out any Fe% deviation in undiluted weld deposits with consumable Vs the filler metal specifications, i.e. AWS A5.14 (GTAW/GMAW) & AWS Sec II, Part-C.
-Procedure qualification tests including simulated corrosion tests such as ASTM G48 Practice A, B are highly recommended to assess the pitting and crevice corrosion resistance performance of test coupons under simulated conditions.
-Spectroscopy (i.e. chemical analysis) of the weld overlay top layer in addition to simulated corrosion tests, will give idea about the relationship between Fe% and Corrosion resistance.
one can also theoretically check the weld metal dilution by formula:-
D(%) = ( CFZ -CFM ) / ( Cs -CFM ) x100
D=% weld dilution
CFZ = elemental composition of the fusion zone (weld)
CFM = elemental composition of the filler metal (wire/strip)
Cs = elemental composition of the substrate (base metal)
The higher the dilution the lower will be the corrosion resistance, as a rule of thumb.
Hope, this information might be helpful to you for the successful procedure qualification of the fabrication involving Inconel 625 overlay to SS 316L components for Sea water application.
We hope this information is useful to you!
Welding Consultant,
Weld Met Advisory Services
References:-
1) M. Gittos, T. G. Gooch Effect of iron dilution on corrosion resistance of Ni-Cr-Mo alloy cladding Published 1996, Materials Science British Corrosion Journal.
2) Elango and S. Balaguru, “Welding Parameters for Inconel 625 Overlay on Carbon Steel using GMAW.” Indian J. Sci. Tech. 8:31 (2015)
3) Evangeline A et al 2019 Cold Metal Arc Transfer (CMT) metal deposition of Inconel 625 super alloy on 316L austenitic stainless steel: microstructural evaluation, corrosion and wear resistance properties, Mater. Res. Express in press https://doi.org/10.1088/2053-1591/ab0a10
4) https://www.twi-global.com/technical-knowledge/published-papers/cra-weld-overlay-influence-of-welding-process-and-parameters-on-dilution-and-corrosion-resistance
5) Huiling Zhou,et al, Effect of Iron Ion on Corrosion Behavior of Inconel 625 in High-Temperature Water,Volume 2020, Article ID 9130362, https://doi.org/10.1155/2020/9130362
6) Kim, J.S., Park, Y.I. & Lee, H.W. Effects of heat input on the pitting resistance of Inconel 625 welds by overlay welding. Met. Mater. Int. 21, 350–355 (2015). https://doi.org/10.1007/s12540-015-4245-9
