Investigation of nickel phospor coat : -As corrosion protection inside water heaters

University essay from Karlstads universitet/Institutionen för ingenjörsvetenskap, fysik och matematik

Author: Björn Wikstrand; [2009]

Keywords: ;

Abstract:

This thesis explores the possibility of alternatives to copper lining inside the water heater

tank in water heaters. The need for an alternative is based on the increasing copper prices the

recent years. The aim of this thesis is to compare three different materials, stainless steel,

copper and a nickel coated plain carbon steel. This comparison is based on a basic corrosion

test and a literature survey to render a merit value for each environment/metal interface. The

testing solution consists of 100ppm Cl

- concentration the specimens are tested in three

different pH levels and at three different temperatures. The specimens have their weight

measured before and after the test.

The 15μm thick nickel coating was performed by Ferroprodukter AB, composing of 9% P

and 91% Ni. The copper and stainless steel specimens are both from Thermia’s actual water

heater tank. The copper lining is made of pure copper and the stainless steel hull is made of

ferritic stainless steel.

The results from the corrosion test are measured in weight change by modulus, |Δw|/w, for

the comparison of the three materials. It was found that stainless steel was the worst

material, performing better at higher temperatures and higher pH , but overall performance is

far from copper and nickel’s corrosion properties. The nickel coating and copper specimen

showed comparable results and perform more or less equally well. Regarding identification

of corrosion mechanics, it may consist of either pitting and general corrosion damage or a

mixture of both. To avoid the fact that weight change may be both negative and positive, the

solution to this was to incorporate a merit value based on the absolute value of the weight

change, divided by the initial weight of the specimen.

In conclusion, stainless steel total weight change by modulus accumulates to 16.072g, nickel

coated specimen accumulates a weight change by modulus equal to 8.544g, important note:

two of the nickel coated specimen account for ~72.4% of the total weight change by modulus

which then accumulates to 2.36g. Copper’s total weight change by modulus was 2.937g thus

the lowest. However, disregarding from the 90캜 regime; stainless steel scores 13.496g,

copper 2.151g, and nickel 1.095g.

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