Turning integer into strings

I have a dataset that has a column called Founded. The founded column contains a variety of years in which a company was founded also the datatype for the column is int64. I have a for loop that is supposed to alter the column based on the if statements

for i in range(2253):
    if type(df['Founded'].iloc[i])== int :
        if df['Founded'].iloc[i]>2000.0 :
            df['Founded'].iloc[i]= 'After 2000'
        elif  1950.0 <df['Founded'].iloc[i]<=2000.0 :
            df['Founded'].iloc[i]= 'Between 1951-2000'
        elif  1900.0 <df['Founded'].iloc[i]<=1950.0 :
            df['Founded'].iloc[i]= 'Between 1901-1950'
        elif df['Founded'].iloc[i]<1900.0 :
            df['Founded'].iloc[i]= 'Before 1900'
    else :
        df['Founded'].iloc[i]=None
    i=i+1

But after I run the code and check the column Founded everthing is changed into Nan. Any suggestions as to what I am do wrong?