Celery (Apium graveolens) was used as natural corrosion inhibitor of carbon steel in 1.0 M HCl. The results of weight
loss method and polarization showed that the inhibition efficiency was increased by increasing the Celery (Apium
graveolens) inhibitor doses and reached the maximum at 500 ppm. The inhibition action of the extract was discussed in
view of Temkin adsorption isotherm. It was found that the adsorption of the extract on Carbon steel surface is a
spontaneous process. The effect of temperature on the IE% was studied; the IE% was decreased with increased
temperature. Also, it was found that the presence of extract increases the activation energy of the corrosion reaction.
Furthermore, the thermodynamic parameters of the adsorption process were calculated.